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Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and RoutingAdobe Systems Incorporated345 Park AveSan JoseCA95110USAfielding@gbiv.comhttp://roy.gbiv.com/greenbytes GmbHHafenweg 16MuensterNW48155Germanyjulian.reschke@greenbytes.dehttp://greenbytes.de/tech/webdav/HTTPbis Working Group
The Hypertext Transfer Protocol (HTTP) is an application-level protocol for
distributed, collaborative, hypertext information systems. HTTP has been in
use by the World Wide Web global information initiative since 1990.
This document provides an overview of HTTP architecture and its associated
terminology, defines the "http" and "https" Uniform Resource Identifier
(URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements,
and describes general security concerns for implementations.
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at
.
The current issues list is at
and related
documents (including fancy diffs) can be found at
.
The changes in this draft are summarized in .
The Hypertext Transfer Protocol (HTTP) is an application-level
request/response protocol that uses extensible semantics and self-descriptive
message payloads for flexible interaction with network-based hypertext
information systems. This document is the first in a series of documents
that collectively form the HTTP/1.1 specification:
RFC xxx1: Message Syntax and RoutingRFC xxx2: Semantics and ContentRFC xxx3: Conditional RequestsRFC xxx4: Range RequestsRFC xxx5: CachingRFC xxx6: Authentication
This HTTP/1.1 specification obsoletes and moves to historic status
RFC 2616, its predecessor
RFC 2068, and
RFC 2145 (on HTTP versioning).
This specification also updates the use of CONNECT to establish a tunnel,
previously defined in RFC 2817,
and defines the "https" URI scheme that was described informally in
RFC 2818.
HTTP is a generic interface protocol for information systems. It is
designed to hide the details of how a service is implemented by presenting
a uniform interface to clients that is independent of the types of
resources provided. Likewise, servers do not need to be aware of each
client's purpose: an HTTP request can be considered in isolation rather
than being associated with a specific type of client or a predetermined
sequence of application steps. The result is a protocol that can be used
effectively in many different contexts and for which implementations can
evolve independently over time.
HTTP is also designed for use as an intermediation protocol for translating
communication to and from non-HTTP information systems.
HTTP proxies and gateways can provide access to alternative information
services by translating their diverse protocols into a hypertext
format that can be viewed and manipulated by clients in the same way
as HTTP services.
One consequence of this flexibility is that the protocol cannot be
defined in terms of what occurs behind the interface. Instead, we
are limited to defining the syntax of communication, the intent
of received communication, and the expected behavior of recipients.
If the communication is considered in isolation, then successful
actions ought to be reflected in corresponding changes to the
observable interface provided by servers. However, since multiple
clients might act in parallel and perhaps at cross-purposes, we
cannot require that such changes be observable beyond the scope
of a single response.
This document describes the architectural elements that are used or
referred to in HTTP, defines the "http" and "https" URI schemes,
describes overall network operation and connection management,
and defines HTTP message framing and forwarding requirements.
Our goal is to define all of the mechanisms necessary for HTTP message
handling that are independent of message semantics, thereby defining the
complete set of requirements for message parsers and
message-forwarding intermediaries.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in .
Conformance criteria and considerations regarding error handling
are defined in .
This specification uses the Augmented Backus-Naur Form (ABNF) notation
of with the list rule extension defined in
. shows
the collected ABNF with the list rule expanded.
The following core rules are included by
reference, as defined in :
ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
DIGIT (decimal 0-9), DQUOTE (double quote),
HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line feed),
OCTET (any 8-bit sequence of data), SP (space), and
VCHAR (any visible character).
As a convention, ABNF rule names prefixed with "obs-" denote
"obsolete" grammar rules that appear for historical reasons.
HTTP was created for the World Wide Web architecture
and has evolved over time to support the scalability needs of a worldwide
hypertext system. Much of that architecture is reflected in the terminology
and syntax productions used to define HTTP.
HTTP is a stateless request/response protocol that operates by exchanging
messages () across a reliable
transport or session-layer
"connection" ().
An HTTP "client" is a program that establishes a connection
to a server for the purpose of sending one or more HTTP requests.
An HTTP "server" is a program that accepts connections
in order to service HTTP requests by sending HTTP responses.
The terms client and server refer only to the roles that
these programs perform for a particular connection. The same program
might act as a client on some connections and a server on others.
We use the term "user agent" to refer to any of the various
client programs that initiate a request, including (but not limited to)
browsers, spiders (web-based robots), command-line tools, native
applications, and mobile apps. The term "origin server" is
used to refer to the program that can originate authoritative responses to
a request. For general requirements, we use the terms
"sender" and "recipient" to refer to any
component that sends or receives, respectively, a given message.
HTTP relies upon the Uniform Resource Identifier (URI)
standard to indicate the target resource
() and relationships between resources.
Messages are passed in a format similar to that used by Internet mail
and the Multipurpose Internet Mail Extensions
(MIME) (see &diff-mime; for the differences
between HTTP and MIME messages).
Most HTTP communication consists of a retrieval request (GET) for
a representation of some resource identified by a URI. In the
simplest case, this might be accomplished via a single bidirectional
connection (===) between the user agent (UA) and the origin server (O).
request >
UA ======================================= O
< response
A client sends an HTTP request to a server in the form of a request
message, beginning with a request-line that includes a method, URI, and
protocol version (),
followed by header fields containing
request modifiers, client information, and representation metadata
(),
an empty line to indicate the end of the header section, and finally
a message body containing the payload body (if any,
).
A server responds to a client's request by sending one or more HTTP
response
messages, each beginning with a status line that
includes the protocol version, a success or error code, and textual
reason phrase (),
possibly followed by header fields containing server
information, resource metadata, and representation metadata
(),
an empty line to indicate the end of the header section, and finally
a message body containing the payload body (if any,
).
A connection might be used for multiple request/response exchanges,
as defined in .
The following example illustrates a typical message exchange for a
GET request on the URI "http://www.example.com/hello.txt":
client request:
GET /hello.txt HTTP/1.1
User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
Host: www.example.com
Accept-Language: en, mi
server response:
HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT
Server: Apache
Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
ETag: "34aa387-d-1568eb00"
Accept-Ranges: bytes
Content-Length:
Vary: Accept-Encoding
Content-Type: text/plain
Hello World!
(Note that the content length includes the trailing CR/LF sequence of the body text)
When considering the design of HTTP, it is easy to fall into a trap of
thinking that all user agents are general-purpose browsers and all origin
servers are large public websites. That is not the case in practice.
Common HTTP user agents include household appliances, stereos, scales,
firmware update scripts, command-line programs, mobile apps,
and communication devices in a multitude of shapes and sizes. Likewise,
common HTTP origin servers include home automation units, configurable
networking components, office machines, autonomous robots, news feeds,
traffic cameras, ad selectors, and video delivery platforms.
The term "user agent" does not imply that there is a human user directly
interacting with the software agent at the time of a request. In many
cases, a user agent is installed or configured to run in the background
and save its results for later inspection (or save only a subset of those
results that might be interesting or erroneous). Spiders, for example, are
typically given a start URI and configured to follow certain behavior while
crawling the Web as a hypertext graph.
The implementation diversity of HTTP means that we cannot assume the
user agent can make interactive suggestions to a user or provide adequate
warning for security or privacy options. In the few cases where this
specification requires reporting of errors to the user, it is acceptable
for such reporting to only be observable in an error console or log file.
Likewise, requirements that an automated action be confirmed by the user
before proceeding can be met via advance configuration choices,
run-time options, or simply not proceeding with the unsafe action.
HTTP enables the use of intermediaries to satisfy requests through
a chain of connections. There are three common forms of HTTP
intermediary: proxy, gateway, and tunnel. In some cases,
a single intermediary might act as an origin server, proxy, gateway,
or tunnel, switching behavior based on the nature of each request.
> > > >
UA =========== A =========== B =========== C =========== O
< < < <
The figure above shows three intermediaries (A, B, and C) between the
user agent and origin server. A request or response message that
travels the whole chain will pass through four separate connections.
Some HTTP communication options
might apply only to the connection with the nearest, non-tunnel
neighbor, only to the end-points of the chain, or to all connections
along the chain. Although the diagram is linear, each participant might
be engaged in multiple, simultaneous communications. For example, B
might be receiving requests from many clients other than A, and/or
forwarding requests to servers other than C, at the same time that it
is handling A's request.
We use the terms "upstream" and "downstream"
to describe various requirements in relation to the directional flow of a
message: all messages flow from upstream to downstream.
Likewise, we use the terms inbound and outbound to refer to
directions in relation to the request path:
"inbound" means toward the origin server and
"outbound" means toward the user agent.
A "proxy" is a message forwarding agent that is selected by the
client, usually via local configuration rules, to receive requests
for some type(s) of absolute URI and attempt to satisfy those
requests via translation through the HTTP interface. Some translations
are minimal, such as for proxy requests for "http" URIs, whereas
other requests might require translation to and from entirely different
application-level protocols. Proxies are often used to group an
organization's HTTP requests through a common intermediary for the
sake of security, annotation services, or shared caching.
An HTTP-to-HTTP proxy is called a "transforming proxy" if it is designed
or configured to modify request or response messages in a semantically
meaningful way (i.e., modifications, beyond those required by normal
HTTP processing, that change the message in a way that would be
significant to the original sender or potentially significant to
downstream recipients). For example, a transforming proxy might be
acting as a shared annotation server (modifying responses to include
references to a local annotation database), a malware filter, a
format transcoder, or an intranet-to-Internet privacy filter. Such
transformations are presumed to be desired by the client (or client
organization) that selected the proxy and are beyond the scope of
this specification. However, when a proxy is not intended to transform
a given message, we use the term "non-transforming proxy" to target
requirements that preserve HTTP message semantics. See &status-203; and
&header-warning; for status and warning codes related to transformations.
A "gateway" (a.k.a., "reverse proxy")
is a receiving agent that acts
as a layer above some other server(s) and translates the received
requests to the underlying server's protocol. Gateways are often
used to encapsulate legacy or untrusted information services, to
improve server performance through "accelerator" caching, and to
enable partitioning or load-balancing of HTTP services across
multiple machines.
A gateway behaves as an origin server on its outbound connection and
as a user agent on its inbound connection.
All HTTP requirements applicable to an origin server
also apply to the outbound communication of a gateway.
A gateway communicates with inbound servers using any protocol that
it desires, including private extensions to HTTP that are outside
the scope of this specification. However, an HTTP-to-HTTP gateway
that wishes to interoperate with third-party HTTP servers &MUST;
conform to HTTP user agent requirements on the gateway's inbound
connection and &MUST; implement the Connection
() and Via
() header fields for both connections.
A "tunnel" acts as a blind relay between two connections
without changing the messages. Once active, a tunnel is not
considered a party to the HTTP communication, though the tunnel might
have been initiated by an HTTP request. A tunnel ceases to exist when
both ends of the relayed connection are closed. Tunnels are used to
extend a virtual connection through an intermediary, such as when
Transport Layer Security (TLS, ) is used to
establish confidential communication through a shared firewall proxy.
The above categories for intermediary only consider those acting as
participants in the HTTP communication. There are also intermediaries
that can act on lower layers of the network protocol stack, filtering or
redirecting HTTP traffic without the knowledge or permission of message
senders. Network intermediaries often introduce security flaws or
interoperability problems by violating HTTP semantics. For example, an
"interception proxy" (also commonly
known as a "transparent proxy" or
"captive portal")
differs from an HTTP proxy because it is not selected by the client.
Instead, an interception proxy filters or redirects outgoing TCP port 80
packets (and occasionally other common port traffic).
Interception proxies are commonly found on public network access points,
as a means of enforcing account subscription prior to allowing use of
non-local Internet services, and within corporate firewalls to enforce
network usage policies.
They are indistinguishable from a man-in-the-middle attack.
HTTP is defined as a stateless protocol, meaning that each request message
can be understood in isolation. Many implementations depend on HTTP's
stateless design in order to reuse proxied connections or dynamically
load-balance requests across multiple servers. Hence, servers &MUST-NOT;
assume that two requests on the same connection are from the same user
agent unless the connection is secured and specific to that agent.
Some non-standard HTTP extensions (e.g., ) have
been known to violate this requirement, resulting in security and
interoperability problems.
A "cache" is a local store of previous response messages and the
subsystem that controls its message storage, retrieval, and deletion.
A cache stores cacheable responses in order to reduce the response
time and network bandwidth consumption on future, equivalent
requests. Any client or server &MAY; employ a cache, though a cache
cannot be used by a server while it is acting as a tunnel.
The effect of a cache is that the request/response chain is shortened
if one of the participants along the chain has a cached response
applicable to that request. The following illustrates the resulting
chain if B has a cached copy of an earlier response from O (via C)
for a request that has not been cached by UA or A.
> >
UA =========== A =========== B - - - - - - C - - - - - - O
< <
A response is "cacheable" if a cache is allowed to store a copy of
the response message for use in answering subsequent requests.
Even when a response is cacheable, there might be additional
constraints placed by the client or by the origin server on when
that cached response can be used for a particular request. HTTP
requirements for cache behavior and cacheable responses are
defined in &caching-overview;.
There are a wide variety of architectures and configurations
of caches deployed across the World Wide Web and
inside large organizations. These include national hierarchies
of proxy caches to save transoceanic bandwidth, collaborative systems that
broadcast or multicast cache entries, archives of pre-fetched cache
entries for use in off-line or high-latency environments, and so on.
This specification targets conformance criteria according to the role of
a participant in HTTP communication. Hence, HTTP requirements are placed
on senders, recipients, clients, servers, user agents, intermediaries,
origin servers, proxies, gateways, or caches, depending on what behavior
is being constrained by the requirement. Additional (social) requirements
are placed on implementations, resource owners, and protocol element
registrations when they apply beyond the scope of a single communication.
The verb "generate" is used instead of "send" where a requirement
differentiates between creating a protocol element and merely forwarding a
received element downstream.
An implementation is considered conformant if it complies with all of the
requirements associated with the roles it partakes in HTTP. Note that
SHOULD-level requirements are relevant here, unless one of the documented
exceptions is applicable.
Conformance applies to both the syntax and semantics of HTTP protocol
elements. A sender &MUST-NOT; generate protocol elements that convey a
meaning that is known by that sender to be false. A sender &MUST-NOT;
generate protocol elements that do not match the grammar defined by the
ABNF rules for those protocol elements that are applicable to the sender's
role. If a received protocol element is processed, the recipient &MUST; be
able to parse any value that would match the ABNF rules for that protocol
element, excluding only those rules not applicable to the recipient's role.
Unless noted otherwise, a recipient &MAY; attempt to recover a usable
protocol element from an invalid construct. HTTP does not define
specific error handling mechanisms except when they have a direct impact
on security, since different applications of the protocol require
different error handling strategies. For example, a Web browser might
wish to transparently recover from a response where the
Location header field doesn't parse according to the ABNF,
whereas a systems control client might consider any form of error recovery
to be dangerous.
HTTP uses a "<major>.<minor>" numbering scheme to indicate
versions of the protocol. This specification defines version "1.1".
The protocol version as a whole indicates the sender's conformance
with the set of requirements laid out in that version's corresponding
specification of HTTP.
The version of an HTTP message is indicated by an HTTP-version field
in the first line of the message. HTTP-version is case-sensitive.
HTTP-version = HTTP-name "/" DIGIT "." DIGITHTTP-name = "HTTP" ; "HTTP", case-sensitive
The HTTP version number consists of two decimal digits separated by a "."
(period or decimal point). The first digit ("major version") indicates the
HTTP messaging syntax, whereas the second digit ("minor version") indicates
the highest minor version to which the sender is
conformant and able to understand for future communication. The minor
version advertises the sender's communication capabilities even when the
sender is only using a backwards-compatible subset of the protocol,
thereby letting the recipient know that more advanced features can
be used in response (by servers) or in future requests (by clients).
When an HTTP/1.1 message is sent to an HTTP/1.0 recipient
or a recipient whose version is unknown,
the HTTP/1.1 message is constructed such that it can be interpreted
as a valid HTTP/1.0 message if all of the newer features are ignored.
This specification places recipient-version requirements on some
new features so that a conformant sender will only use compatible
features until it has determined, through configuration or the
receipt of a message, that the recipient supports HTTP/1.1.
The interpretation of a header field does not change between minor
versions of the same major HTTP version, though the default
behavior of a recipient in the absence of such a field can change.
Unless specified otherwise, header fields defined in HTTP/1.1 are
defined for all versions of HTTP/1.x. In particular, the Host
and Connection header fields ought to be implemented by all
HTTP/1.x implementations whether or not they advertise conformance with
HTTP/1.1.
New header fields can be defined such that, when they are
understood by a recipient, they might override or enhance the
interpretation of previously defined header fields. When an
implementation receives an unrecognized header field, the recipient
&MUST; ignore that header field for local processing regardless of
the message's HTTP version. An unrecognized header field received
by a proxy &MUST; be forwarded downstream unless the header field's
field-name is listed in the message's Connection header field
(see ).
These requirements allow HTTP's functionality to be enhanced without
requiring prior update of deployed intermediaries.
Intermediaries that process HTTP messages (i.e., all intermediaries
other than those acting as tunnels) &MUST; send their own HTTP-version
in forwarded messages. In other words, they &MUST-NOT; blindly
forward the first line of an HTTP message without ensuring that the
protocol version in that message matches a version to which that
intermediary is conformant for both the receiving and
sending of messages. Forwarding an HTTP message without rewriting
the HTTP-version might result in communication errors when downstream
recipients use the message sender's version to determine what features
are safe to use for later communication with that sender.
An HTTP client &SHOULD; send a request version equal to the highest
version to which the client is conformant and
whose major version is no higher than the highest version supported
by the server, if this is known. An HTTP client &MUST-NOT; send a
version to which it is not conformant.
An HTTP client &MAY; send a lower request version if it is known that
the server incorrectly implements the HTTP specification, but only
after the client has attempted at least one normal request and determined
from the response status or header fields (e.g., Server) that
the server improperly handles higher request versions.
An HTTP server &SHOULD; send a response version equal to the highest
version to which the server is conformant and
whose major version is less than or equal to the one received in the
request. An HTTP server &MUST-NOT; send a version to which it is not
conformant. A server &MAY; send a 505 (HTTP Version Not
Supported) response if it cannot send a response using the
major version used in the client's request.
An HTTP server &MAY; send an HTTP/1.0 response to an HTTP/1.0 request
if it is known or suspected that the client incorrectly implements the
HTTP specification and is incapable of correctly processing later
version responses, such as when a client fails to parse the version
number correctly or when an intermediary is known to blindly forward
the HTTP-version even when it doesn't conform to the given minor
version of the protocol. Such protocol downgrades &SHOULD-NOT; be
performed unless triggered by specific client attributes, such as when
one or more of the request header fields (e.g., User-Agent)
uniquely match the values sent by a client known to be in error.
The intention of HTTP's versioning design is that the major number
will only be incremented if an incompatible message syntax is
introduced, and that the minor number will only be incremented when
changes made to the protocol have the effect of adding to the message
semantics or implying additional capabilities of the sender. However,
the minor version was not incremented for the changes introduced between
and , and this revision
has specifically avoiding any such changes to the protocol.
Uniform Resource Identifiers (URIs) are used
throughout HTTP as the means for identifying resources (&resource;).
URI references are used to target requests, indicate redirects, and define
relationships.
This specification adopts the definitions of "URI-reference",
"absolute-URI", "relative-part", "port", "host",
"path-abempty", "query", "segment", and "authority" from the
URI generic syntax.
In addition, we define an "absolute-path" rule (that differs from
RFC 3986's "path-absolute" in that it allows a leading "//")
and a "partial-URI" rule for protocol elements
that allow a relative URI but not a fragment.
URI-reference = <URI-reference, defined in >
absolute-URI = <absolute-URI, defined in >
relative-part = <relative-part, defined in >
authority = <authority, defined in >
path-abempty = <path-abempty, defined in >
port = <port, defined in >
query = <query, defined in >
segment = <segment, defined in >
uri-host = <host, defined in >
absolute-path = 1*( "/" segment )
partial-URI = relative-part [ "?" query ]
Each protocol element in HTTP that allows a URI reference will indicate
in its ABNF production whether the element allows any form of reference
(URI-reference), only a URI in absolute form (absolute-URI), only the
path and optional query components, or some combination of the above.
Unless otherwise indicated, URI references are parsed
relative to the effective request URI
().
The "http" URI scheme is hereby defined for the purpose of minting
identifiers according to their association with the hierarchical
namespace governed by a potential HTTP origin server listening for
TCP connections on a given port.
http-URI = "http:" "//" authoritypath-abempty [ "?" query ]
The HTTP origin server is identified by the generic syntax's
authority component, which includes a host identifier
and optional TCP port ().
The remainder of the URI, consisting of both the hierarchical path
component and optional query component, serves as an identifier for
a potential resource within that origin server's name space.
If the host identifier is provided as an IP address,
then the origin server is any listener on the indicated TCP port at
that IP address. If host is a registered name, then that name is
considered an indirect identifier and the recipient might use a name
resolution service, such as DNS, to find the address of a listener
for that host.
The host &MUST-NOT; be empty; if an "http" URI is received with an
empty host, then it &MUST; be rejected as invalid.
If the port subcomponent is empty or not given, then TCP port 80 is
assumed (the default reserved port for WWW services).
Regardless of the form of host identifier, access to that host is not
implied by the mere presence of its name or address. The host might or might
not exist and, even when it does exist, might or might not be running an
HTTP server or listening to the indicated port. The "http" URI scheme
makes use of the delegated nature of Internet names and addresses to
establish a naming authority (whatever entity has the ability to place
an HTTP server at that Internet name or address) and allows that
authority to determine which names are valid and how they might be used.
When an "http" URI is used within a context that calls for access to the
indicated resource, a client &MAY; attempt access by resolving
the host to an IP address, establishing a TCP connection to that address
on the indicated port, and sending an HTTP request message
() containing the URI's identifying data
() to the server.
If the server responds to that request with a non-interim HTTP response
message, as described in &status-codes;, then that response
is considered an authoritative answer to the client's request.
Although HTTP is independent of the transport protocol, the "http"
scheme is specific to TCP-based services because the name delegation
process depends on TCP for establishing authority.
An HTTP service based on some other underlying connection protocol
would presumably be identified using a different URI scheme, just as
the "https" scheme (below) is used for resources that require an
end-to-end secured connection. Other protocols might also be used to
provide access to "http" identified resources — it is only the
authoritative interface used for mapping the namespace that is
specific to TCP.
The URI generic syntax for authority also includes a deprecated
userinfo subcomponent ()
for including user authentication information in the URI. Some
implementations make use of the userinfo component for internal
configuration of authentication information, such as within command
invocation options, configuration files, or bookmark lists, even
though such usage might expose a user identifier or password.
Senders &MUST; exclude the userinfo subcomponent (and its "@"
delimiter) when an "http" URI is transmitted within a message as a
request target or header field value.
Recipients of an "http" URI reference &SHOULD; parse for userinfo and
treat its presence as an error, since it is likely being used to obscure
the authority for the sake of phishing attacks.
The "https" URI scheme is hereby defined for the purpose of minting
identifiers according to their association with the hierarchical
namespace governed by a potential HTTP origin server listening to a
given TCP port for TLS-secured connections .
All of the requirements listed above for the "http" scheme are also
requirements for the "https" scheme, except that a default TCP port
of 443 is assumed if the port subcomponent is empty or not given,
and the TCP connection &MUST; be secured, end-to-end, through the
use of strong encryption prior to sending the first HTTP request.
https-URI = "https:" "//" authoritypath-abempty [ "?" query ]
Resources made available via the "https" scheme have no shared
identity with the "http" scheme even if their resource identifiers
indicate the same authority (the same host listening to the same
TCP port). They are distinct name spaces and are considered to be
distinct origin servers. However, an extension to HTTP that is
defined to apply to entire host domains, such as the Cookie protocol
, can allow information
set by one service to impact communication with other services
within a matching group of host domains.
The process for authoritative access to an "https" identified
resource is defined in .
Since the "http" and "https" schemes conform to the URI generic syntax,
such URIs are normalized and compared according to the algorithm defined
in , using the defaults
described above for each scheme.
If the port is equal to the default port for a scheme, the normal form is
to elide the port subcomponent. When not being used in absolute form as the
request target of an OPTIONS request, an empty path component is equivalent
to an absolute path of "/", so the normal form is to provide a path of "/"
instead. The scheme and host are case-insensitive and normally provided in
lowercase; all other components are compared in a case-sensitive manner.
Characters other than those in the "reserved" set are equivalent to their
percent-encoded octets (see ): the normal form is to not encode them.
For example, the following three URIs are equivalent:
http://example.com:80/~smith/home.html
http://EXAMPLE.com/%7Esmith/home.html
http://EXAMPLE.com:/%7esmith/home.html
All HTTP/1.1 messages consist of a start-line followed by a sequence of
octets in a format similar to the Internet Message Format
: zero or more header fields (collectively
referred to as the "headers" or the "header section"), an empty line
indicating the end of the header section, and an optional message body.
HTTP-message = start-line
*( header-fieldCRLF )
CRLF
[ message-body ]
The normal procedure for parsing an HTTP message is to read the
start-line into a structure, read each header field into a hash
table by field name until the empty line, and then use the parsed
data to determine if a message body is expected. If a message body
has been indicated, then it is read as a stream until an amount
of octets equal to the message body length is read or the connection
is closed.
Recipients &MUST; parse an HTTP message as a sequence of octets in an
encoding that is a superset of US-ASCII .
Parsing an HTTP message as a stream of Unicode characters, without regard
for the specific encoding, creates security vulnerabilities due to the
varying ways that string processing libraries handle invalid multibyte
character sequences that contain the octet LF (%x0A). String-based
parsers can only be safely used within protocol elements after the element
has been extracted from the message, such as within a header field-value
after message parsing has delineated the individual fields.
An HTTP message can be parsed as a stream for incremental processing or
forwarding downstream. However, recipients cannot rely on incremental
delivery of partial messages, since some implementations will buffer or
delay message forwarding for the sake of network efficiency, security
checks, or payload transformations.
An HTTP message can either be a request from client to server or a
response from server to client. Syntactically, the two types of message
differ only in the start-line, which is either a request-line (for requests)
or a status-line (for responses), and in the algorithm for determining
the length of the message body ().
In theory, a client could receive requests and a server could receive
responses, distinguishing them by their different start-line formats,
but in practice servers are implemented to only expect a request
(a response is interpreted as an unknown or invalid request method)
and clients are implemented to only expect a response.
start-line = request-line / status-line
A sender &MUST-NOT; send whitespace between the start-line and
the first header field. The presence of such whitespace in a request
might be an attempt to trick a server into ignoring that field or
processing the line after it as a new request, either of which might
result in a security vulnerability if other implementations within
the request chain interpret the same message differently.
Likewise, the presence of such whitespace in a response might be
ignored by some clients or cause others to cease parsing.
A recipient that receives whitespace between the start-line and
the first header field &MUST; either reject the message as invalid or
consume each whitespace-preceded line without further processing of it
(i.e., ignore the entire line, along with any subsequent lines preceded
by whitespace, until a properly formed header field is received or the
header block is terminated).
A request-line begins with a method token, followed by a single
space (SP), the request-target, another single space (SP), the
protocol version, and ending with CRLF.
request-line = methodSPrequest-targetSPHTTP-versionCRLF
The method token indicates the request method to be performed on the
target resource. The request method is case-sensitive.
method = token
The methods defined by this specification can be found in
&methods;, along with information regarding the HTTP method registry
and considerations for defining new methods.
The request-target identifies the target resource upon which to apply
the request, as defined in .
No whitespace is allowed inside the method, request-target, and
protocol version. Hence, recipients typically parse the request-line
into its component parts by splitting on whitespace
(see ).
Unfortunately, some user agents fail to properly encode hypertext
references that have embedded whitespace, sending the characters directly
instead of properly encoding or excluding the disallowed characters.
Recipients of an invalid request-line &SHOULD; respond with either a
400 (Bad Request) error or a 301 (Moved Permanently)
redirect with the request-target properly encoded. Recipients &SHOULD-NOT;
attempt to autocorrect and then process the request without a redirect,
since the invalid request-line might be deliberately crafted to bypass
security filters along the request chain.
HTTP does not place a pre-defined limit on the length of a request-line.
A server that receives a method longer than any that it implements
&SHOULD; respond with a 501 (Not Implemented) status code.
A server &MUST; be prepared to receive URIs of unbounded length and
respond with the 414 (URI Too Long) status code if the received
request-target would be longer than the server wishes to handle
(see &status-414;).
Various ad-hoc limitations on request-line length are found in practice.
It is &RECOMMENDED; that all HTTP senders and recipients support, at a
minimum, request-line lengths of 8000 octets.
The first line of a response message is the status-line, consisting
of the protocol version, a space (SP), the status code, another space,
a possibly-empty textual phrase describing the status code, and
ending with CRLF.
status-line = HTTP-versionSPstatus-codeSPreason-phraseCRLF
The status-code element is a 3-digit integer code describing the
result of the server's attempt to understand and satisfy the client's
corresponding request. The rest of the response message is to be
interpreted in light of the semantics defined for that status code.
See &status-codes; for information about the semantics of status codes,
including the classes of status code (indicated by the first digit),
the status codes defined by this specification, considerations for the
definition of new status codes, and the IANA registry.
status-code = 3DIGIT
The reason-phrase element exists for the sole purpose of providing a
textual description associated with the numeric status code, mostly
out of deference to earlier Internet application protocols that were more
frequently used with interactive text clients. A client &SHOULD; ignore
the reason-phrase content.
reason-phrase = *( HTAB / SP / VCHAR / obs-text )
Each HTTP header field consists of a case-insensitive field name
followed by a colon (":"), optional whitespace, and the field value.
header-field = field-name ":" OWSfield-valueBWSfield-name = tokenfield-value = *( field-content / obs-fold )
field-content = *( HTAB / SP / VCHAR / obs-text )
obs-fold = CRLF ( SP / HTAB )
; obsolete line folding
; see
The field-name token labels the corresponding field-value as having the
semantics defined by that header field. For example, the Date
header field is defined in &header-date; as containing the origination
timestamp for the message in which it appears.
HTTP header fields are fully extensible: there is no limit on the
introduction of new field names, each presumably defining new semantics,
nor on the number of header fields used in a given message. Existing
fields are defined in each part of this specification and in many other
specifications outside the core standard.
New header fields can be introduced without changing the protocol version
if their defined semantics allow them to be safely ignored by recipients
that do not recognize them.
New HTTP header fields ought to be be registered with IANA in the
Message Header Field Registry, as described in &iana-header-registry;.
A proxy &MUST; forward unrecognized header fields unless the
field-name is listed in the Connection header field
() or the proxy is specifically
configured to block, or otherwise transform, such fields.
Other recipients &SHOULD; ignore unrecognized header fields.
The order in which header fields with differing field names are
received is not significant. However, it is "good practice" to send
header fields that contain control data first, such as Host
on requests and Date on responses, so that implementations
can decide when not to handle a message as early as possible. A server
&MUST; wait until the entire header section is received before interpreting
a request message, since later header fields might include conditionals,
authentication credentials, or deliberately misleading duplicate
header fields that would impact request processing.
A sender &MUST-NOT; generate multiple header fields with the same field
name in a message unless either the entire field value for that
header field is defined as a comma-separated list [i.e., #(values)]
or the header field is a well-known exception (as noted below).
Multiple header fields with the same field name can be combined into
one "field-name: field-value" pair, without changing the semantics of the
message, by appending each subsequent field value to the combined
field value in order, separated by a comma. The order in which
header fields with the same field name are received is therefore
significant to the interpretation of the combined field value;
a proxy &MUST-NOT; change the order of these field values when
forwarding a message.
&Note; In practice, the "Set-Cookie" header field ()
often appears multiple times in a response message and does not use the
list syntax, violating the above requirements on multiple header fields
with the same name. Since it cannot be combined into a single field-value,
recipients ought to handle "Set-Cookie" as a special case while processing
header fields. (See Appendix A.2.3 of for details.)
This specification uses three rules to denote the use of linear
whitespace: OWS (optional whitespace), RWS (required whitespace), and
BWS ("bad" whitespace).
The OWS rule is used where zero or more linear whitespace octets might
appear. OWS &SHOULD; either not be generated or be generated as a single
SP. Multiple OWS octets that occur within field-content &SHOULD; either
be replaced with a single SP or transformed to all SP octets (each
octet other than SP replaced with SP) before interpreting the field value
or forwarding the message downstream.
RWS is used when at least one linear whitespace octet is required to
separate field tokens. RWS &SHOULD; be generated as a single SP.
Multiple RWS octets that occur within field-content &SHOULD; either
be replaced with a single SP or transformed to all SP octets before
interpreting the field value or forwarding the message downstream.
BWS is used where the grammar allows optional whitespace, for historical
reasons, but senders &SHOULD-NOT; generate it in messages;
recipients &MUST; accept such bad optional whitespace and remove it before
interpreting the field value or forwarding the message downstream.
OWS = *( SP / HTAB )
; optional whitespace
RWS = 1*( SP / HTAB )
; required whitespace
BWS = OWS
; "bad" whitespace
No whitespace is allowed between the header field-name and colon.
In the past, differences in the handling of such whitespace have led to
security vulnerabilities in request routing and response handling.
A server &MUST; reject any received request message that contains
whitespace between a header field-name and colon with a response code of
400 (Bad Request). A proxy &MUST; remove any such whitespace
from a response message before forwarding the message downstream.
A field value is preceded by optional whitespace (OWS); a single SP is
preferred. The field value does not include any leading or trailing white
space: OWS occurring before the first non-whitespace octet of the
field value or after the last non-whitespace octet of the field value
is ignored and &SHOULD; be removed before further processing (as this does
not change the meaning of the header field).
Historically, HTTP header field values could be extended over multiple
lines by preceding each extra line with at least one space or horizontal
tab (obs-fold). This specification deprecates such line folding except
within the message/http media type
().
Senders &MUST-NOT; generate messages that include line folding
(i.e., that contain any field-value that contains a match to the
obs-fold rule) unless the message is intended for packaging
within the message/http media type. When an obs-fold is
received in a message, recipients &MUST; do one of:
accept the message and replace any embedded obs-fold
whitespace with either a single SP or a matching
number of SP octets (to avoid buffer copying) prior to
interpreting the field value or forwarding the message
downstream;if it is a request, reject the message by sending a
400 (Bad Request) response with a representation
explaining that obsolete line folding is unacceptable; or,if it is a response, discard the message and generate a
502 (Bad Gateway) response with a representation
explaining that unacceptable line folding was received.
Recipients that choose not to implement obs-fold processing
(as described above) &MUST-NOT; accept messages containing header fields
with leading whitespace, as this can expose them to attacks that exploit
this difference in processing.
Historically, HTTP has allowed field content with text in the ISO-8859-1
charset, supporting other charsets only
through use of encoding.
In practice, most HTTP header field values use only a subset of the
US-ASCII charset . Newly defined
header fields &SHOULD; limit their field values to US-ASCII octets.
Recipients &SHOULD; treat other octets in field content (obs-text) as
opaque data.
HTTP does not place a pre-defined limit on the length of each header field
or on the length of the header block as a whole. Various ad-hoc
limitations on individual header field length are found in practice,
often depending on the specific field semantics.
A server &MUST; be prepared to receive request header fields of unbounded
length and respond with an appropriate 4xx (Client Error)
status code if the received header field(s) are larger than the server
wishes to process.
A client &MUST; be prepared to receive response header fields of unbounded
length. A client &MAY; discard or truncate received header fields that are
larger than the client wishes to process if the field semantics are such
that the dropped value(s) can be safely ignored without changing the
response semantics.
Many HTTP header field values consist of words (token or quoted-string)
separated by whitespace or special characters. These special characters
&MUST; be in a quoted string to be used within a parameter value (as defined
in ).
word = token / quoted-stringtoken = 1*tchartchar = "!" / "#" / "$" / "%" / "&" / "'" / "*"
/ "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
/ DIGIT / ALPHA
; any VCHAR, except specialspecial = "(" / ")" / "<" / ">" / "@" / ","
/ ";" / ":" / "\" / DQUOTE / "/" / "["
/ "]" / "?" / "=" / "{" / "}"
A string of text is parsed as a single word if it is quoted using
double-quote marks.
quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTEqdtext = HTAB / SP /%x21 / %x23-5B / %x5D-7E / obs-textobs-text = %x80-FF
The backslash octet ("\") can be used as a single-octet
quoting mechanism within quoted-string constructs:
quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )
Recipients that process the value of a quoted-string &MUST; handle a
quoted-pair as if it were replaced by the octet following the backslash.
Senders &SHOULD-NOT; generate a quoted-pair in a quoted-string except where
necessary to quote DQUOTE and backslash octets occurring within that string.
Comments can be included in some HTTP header fields by surrounding
the comment text with parentheses. Comments are only allowed in
fields containing "comment" as part of their field value definition.
comment = "(" *( ctext / quoted-cpair / comment ) ")"
ctext = HTAB / SP / %x21-27 / %x2A-5B / %x5D-7E / obs-text
The backslash octet ("\") can be used as a single-octet
quoting mechanism within comment constructs:
quoted-cpair = "\" ( HTAB / SP / VCHAR / obs-text )
Senders &SHOULD-NOT; escape octets in comments that do not require escaping
(i.e., other than the backslash octet "\" and the parentheses "(" and ")").
The message body (if any) of an HTTP message is used to carry the
payload body of that request or response. The message body is
identical to the payload body unless a transfer coding has been
applied, as described in .
message-body = *OCTET
The rules for when a message body is allowed in a message differ for
requests and responses.
The presence of a message body in a request is signaled by a
Content-Length or Transfer-Encoding header
field. Request message framing is independent of method semantics,
even if the method does not define any use for a message body.
The presence of a message body in a response depends on both
the request method to which it is responding and the response
status code ().
Responses to the HEAD request method never include a message body
because the associated response header fields (e.g.,
Transfer-Encoding, Content-Length, etc.),
if present, indicate only what their values would have been if the request
method had been GET (&HEAD;).
2xx (Successful) responses to CONNECT switch to tunnel
mode instead of having a message body (&CONNECT;).
All 1xx (Informational), 204 (No Content), and
304 (Not Modified) responses do not include a message body.
All other responses do include a message body, although the body
might be of zero length.
The Transfer-Encoding header field lists the transfer coding names
corresponding to the sequence of transfer codings that have been
(or will be) applied to the payload body in order to form the message body.
Transfer codings are defined in .
Transfer-Encoding = 1#transfer-coding
Transfer-Encoding is analogous to the Content-Transfer-Encoding field of
MIME, which was designed to enable safe transport of binary data over a
7-bit transport service ().
However, safe transport has a different focus for an 8bit-clean transfer
protocol. In HTTP's case, Transfer-Encoding is primarily intended to
accurately delimit a dynamically generated payload and to distinguish
payload encodings that are only applied for transport efficiency or
security from those that are characteristics of the selected resource.
All HTTP/1.1 recipients &MUST; implement the chunked transfer coding
() because it plays a crucial role in
framing messages when the payload body size is not known in advance.
If chunked is applied to a payload body, the sender &MUST-NOT; apply
chunked more than once (i.e., chunking an already chunked message is not
allowed).
If any transfer coding is applied to a request payload body, the
sender &MUST; apply chunked as the final transfer coding to ensure that
the message is properly framed.
If any transfer coding is applied to a response payload body, the
sender &MUST; either apply chunked as the final transfer coding or
terminate the message by closing the connection.
For example,
Transfer-Encoding: gzip, chunked
indicates that the payload body has been compressed using the gzip
coding and then chunked using the chunked coding while forming the
message body.
Unlike Content-Encoding (&content-codings;),
Transfer-Encoding is a property of the message, not of the representation, and
any recipient along the request/response chain &MAY; decode the received
transfer coding(s) or apply additional transfer coding(s) to the message
body, assuming that corresponding changes are made to the Transfer-Encoding
field-value. Additional information about the encoding parameters &MAY; be
provided by other header fields not defined by this specification.
Transfer-Encoding &MAY; be sent in a response to a HEAD request or in a
304 (Not Modified) response (&status-304;) to a GET request,
neither of which includes a message body,
to indicate that the origin server would have applied a transfer coding
to the message body if the request had been an unconditional GET.
This indication is not required, however, because any recipient on
the response chain (including the origin server) can remove transfer
codings when they are not needed.
Transfer-Encoding was added in HTTP/1.1. It is generally assumed that
implementations advertising only HTTP/1.0 support will not understand
how to process a transfer-encoded payload.
A client &MUST-NOT; send a request containing Transfer-Encoding unless it
knows the server will handle HTTP/1.1 (or later) requests; such knowledge
might be in the form of specific user configuration or by remembering the
version of a prior received response.
A server &MUST-NOT; send a response containing Transfer-Encoding unless
the corresponding request indicates HTTP/1.1 (or later).
A server that receives a request message with a transfer coding it does
not understand &SHOULD; respond with 501 (Not Implemented).
When a message does not have a Transfer-Encoding header
field, a Content-Length header field can provide the anticipated size,
as a decimal number of octets, for a potential payload body.
For messages that do include a payload body, the Content-Length field-value
provides the framing information necessary for determining where the body
(and message) ends. For messages that do not include a payload body, the
Content-Length indicates the size of the selected representation
(&representation;).
Content-Length = 1*DIGIT
An example is
Content-Length: 3495
A sender &MUST-NOT; send a Content-Length header field in any message that
contains a Transfer-Encoding header field.
A user agent &SHOULD; send a Content-Length in a request message when no
Transfer-Encoding is sent and the request method defines
a meaning for an enclosed payload body. For example, a Content-Length
header field is normally sent in a POST request even when the value is
0 (indicating an empty payload body). A user agent &SHOULD-NOT; send a
Content-Length header field when the request message does not contain a
payload body and the method semantics do not anticipate such a body.
A server &MAY; send a Content-Length header field in a response to a HEAD
request (&HEAD;); a server &MUST-NOT; send Content-Length in such a
response unless its field-value equals the decimal number of octets that
would have been sent in the payload body of a response if the same
request had used the GET method.
A server &MAY; send a Content-Length header field in a
304 (Not Modified) response to a conditional GET request
(&status-304;); a server &MUST-NOT; send Content-Length in such a
response unless its field-value equals the decimal number of octets that
would have been sent in the payload body of a 200 (OK)
response to the same request.
A server &MUST-NOT; send a Content-Length header field in any response
with a status code of
1xx (Informational) or 204 (No Content).
A server &SHOULD-NOT; send a Content-Length header field in any
2xx (Successful) response to a CONNECT request (&CONNECT;).
Aside from the cases defined above, in the absence of Transfer-Encoding,
an origin server &SHOULD; send a Content-Length header field when the
payload body size is known prior to sending the complete header block.
This will allow downstream recipients to measure transfer progress,
know when a received message is complete, and potentially reuse the
connection for additional requests.
Any Content-Length field value greater than or equal to zero is valid.
Since there is no predefined limit to the length of a payload,
recipients &SHOULD; anticipate potentially large decimal numerals and
prevent parsing errors due to integer conversion overflows
().
If a message is received that has multiple Content-Length header fields
with field-values consisting of the same decimal value, or a single
Content-Length header field with a field value containing a list of
identical decimal values (e.g., "Content-Length: 42, 42"), indicating that
duplicate Content-Length header fields have been generated or combined by an
upstream message processor, then the recipient &MUST; either reject the
message as invalid or replace the duplicated field-values with a single
valid Content-Length field containing that decimal value prior to
determining the message body length.
&Note; HTTP's use of Content-Length for message framing differs
significantly from the same field's use in MIME, where it is an optional
field used only within the "message/external-body" media-type.
The length of a message body is determined by one of the following
(in order of precedence):
Any response to a HEAD request and any response with a
1xx (Informational), 204 (No Content), or
304 (Not Modified) status code is always
terminated by the first empty line after the header fields, regardless of
the header fields present in the message, and thus cannot contain a
message body.
Any 2xx (Successful) response to a CONNECT request implies that the
connection will become a tunnel immediately after the empty line that
concludes the header fields. A client &MUST; ignore any
Content-Length or Transfer-Encoding header
fields received in such a message.
If a Transfer-Encoding header field is present
and the chunked transfer coding ()
is the final encoding, the message body length is determined by reading
and decoding the chunked data until the transfer coding indicates the
data is complete.
If a Transfer-Encoding header field is present in a
response and the chunked transfer coding is not the final encoding, the
message body length is determined by reading the connection until it is
closed by the server.
If a Transfer-Encoding header field is present in a request and the
chunked transfer coding is not the final encoding, the message body
length cannot be determined reliably; the server &MUST; respond with
the 400 (Bad Request) status code and then close the connection.
If a message is received with both a Transfer-Encoding
and a Content-Length header field, the Transfer-Encoding
overrides the Content-Length. Such a message might indicate an attempt
to perform request or response smuggling (bypass of security-related
checks on message routing or content) and thus ought to be handled as
an error. A sender &MUST; remove the received Content-Length field
prior to forwarding such a message downstream.
If a message is received without Transfer-Encoding and with
either multiple Content-Length header fields having
differing field-values or a single Content-Length header field having an
invalid value, then the message framing is invalid and &MUST; be treated
as an error to prevent request or response smuggling.
If this is a request message, the server &MUST; respond with
a 400 (Bad Request) status code and then close the connection.
If this is a response message received by a proxy, the proxy
&MUST; discard the received response, send a 502 (Bad Gateway)
status code as its downstream response, and then close the connection.
If this is a response message received by a user agent, it &MUST; be
treated as an error by discarding the message and closing the connection.
If a valid Content-Length header field is present without
Transfer-Encoding, its decimal value defines the
expected message body length in octets.
If the sender closes the connection or the recipient times out before the
indicated number of octets are received, the recipient &MUST; consider
the message to be incomplete and close the connection.
If this is a request message and none of the above are true, then the
message body length is zero (no message body is present).
Otherwise, this is a response message without a declared message body
length, so the message body length is determined by the number of octets
received prior to the server closing the connection.
Since there is no way to distinguish a successfully completed,
close-delimited message from a partially-received message interrupted
by network failure, a server &SHOULD; use encoding or
length-delimited messages whenever possible. The close-delimiting
feature exists primarily for backwards compatibility with HTTP/1.0.
A server &MAY; reject a request that contains a message body but
not a Content-Length by responding with
411 (Length Required).
Unless a transfer coding other than chunked has been applied,
a client that sends a request containing a message body &SHOULD;
use a valid Content-Length header field if the message body
length is known in advance, rather than the chunked transfer coding, since some
existing services respond to chunked with a 411 (Length Required)
status code even though they understand the chunked transfer coding. This
is typically because such services are implemented via a gateway that
requires a content-length in advance of being called and the server
is unable or unwilling to buffer the entire request before processing.
A user agent that sends a request containing a message body &MUST; send a
valid Content-Length header field if it does not know the
server will handle HTTP/1.1 (or later) requests; such knowledge can be in
the form of specific user configuration or by remembering the version of a
prior received response.
If the final response to the last request on a connection has been
completely received and there remains additional data to read, a user agent
&MAY; discard the remaining data or attempt to determine if that data
belongs as part of the prior response body, which might be the case if the
prior message's Content-Length value is incorrect. A client &MUST-NOT;
process, cache, or forward such extra data as a separate response, since
such behavior would be vulnerable to cache poisoning.
A server that receives an incomplete request message, usually due to a
canceled request or a triggered time-out exception, &MAY; send an error
response prior to closing the connection.
A client that receives an incomplete response message, which can occur
when a connection is closed prematurely or when decoding a supposedly
chunked transfer coding fails, &MUST; record the message as incomplete.
Cache requirements for incomplete responses are defined in
&cache-incomplete;.
If a response terminates in the middle of the header block (before the
empty line is received) and the status code might rely on header fields to
convey the full meaning of the response, then the client cannot assume
that meaning has been conveyed; the client might need to repeat the
request in order to determine what action to take next.
A message body that uses the chunked transfer coding is
incomplete if the zero-sized chunk that terminates the encoding has not
been received. A message that uses a valid Content-Length is
incomplete if the size of the message body received (in octets) is less than
the value given by Content-Length. A response that has neither chunked
transfer coding nor Content-Length is terminated by closure of the
connection, and thus is considered complete regardless of the number of
message body octets received, provided that the header block was received
intact.
Older HTTP/1.0 user agent implementations might send an extra CRLF
after a POST request as a lame workaround for some early server
applications that failed to read message body content that was
not terminated by a line-ending. An HTTP/1.1 user agent &MUST-NOT;
preface or follow a request with an extra CRLF. If terminating
the request message body with a line-ending is desired, then the
user agent &MUST; count the terminating CRLF octets as part of the
message body length.
In the interest of robustness, servers &SHOULD; ignore at least one
empty line received where a request-line is expected. In other words, if
a server is reading the protocol stream at the beginning of a
message and receives a CRLF first, the server &SHOULD; ignore the CRLF.
Although the line terminator for the start-line and header
fields is the sequence CRLF, recipients &MAY; recognize a
single LF as a line terminator and ignore any preceding CR.
Although the request-line and status-line grammar rules require that each
of the component elements be separated by a single SP octet, recipients
&MAY; instead parse on whitespace-delimited word boundaries and, aside
from the CRLF terminator, treat any form of whitespace as the SP separator
while ignoring preceding or trailing whitespace;
such whitespace includes one or more of the following octets:
SP, HTAB, VT (%x0B), FF (%x0C), or bare CR.
When a server listening only for HTTP request messages, or processing
what appears from the start-line to be an HTTP request message,
receives a sequence of octets that does not match the HTTP-message
grammar aside from the robustness exceptions listed above, the
server &SHOULD; respond with a 400 (Bad Request) response.
Transfer coding names are used to indicate an encoding
transformation that has been, can be, or might need to be applied to a
payload body in order to ensure "safe transport" through the network.
This differs from a content coding in that the transfer coding is a
property of the message rather than a property of the representation
that is being transferred.
transfer-coding = "chunked" ;
/ "compress" ;
/ "deflate" ;
/ "gzip" ;
/ transfer-extensiontransfer-extension = token *( OWS ";" OWStransfer-parameter )
Parameters are in the form of attribute/value pairs.
transfer-parameter = attributeBWS "=" BWSvalueattribute = tokenvalue = word
All transfer-coding names are case-insensitive and ought to be registered
within the HTTP Transfer Coding registry, as defined in
.
They are used in the TE () and
Transfer-Encoding ()
header fields.
The chunked transfer coding modifies the body of a message in order to
transfer it as a series of chunks, each with its own size indicator,
followed by an &OPTIONAL; trailer containing header fields. This
allows dynamically generated content to be transferred along with the
information necessary for the recipient to verify that it has
received the full message.
chunked-body = *chunklast-chunktrailer-partCRLFchunk = chunk-size [ chunk-ext ] CRLFchunk-dataCRLFchunk-size = 1*HEXDIGlast-chunk = 1*("0") [ chunk-ext ] CRLFchunk-ext = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
chunk-ext-name = tokenchunk-ext-val = token / quoted-str-nfchunk-data = 1*OCTET ; a sequence of chunk-size octets
trailer-part = *( header-fieldCRLF )
quoted-str-nf = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
; like quoted-string, but disallowing line folding
qdtext-nf = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text
Chunk extensions within the chunked transfer coding are deprecated.
Senders &SHOULD-NOT; send chunk-ext.
Definition of new chunk extensions is discouraged.
The chunk-size field is a string of hex digits indicating the size of
the chunk-data in octets. The chunked transfer coding is complete when a
chunk with a chunk-size of zero is received, possibly followed by a
trailer, and finally terminated by an empty line.
A trailer allows the sender to include additional fields at the end of a
chunked message in order to supply metadata that might be dynamically
generated while the message body is sent, such as a message integrity
check, digital signature, or post-processing status.
The trailer &MUST-NOT; contain fields that need to be known before a
recipient processes the body, such as Transfer-Encoding,
Content-Length, and Trailer.
When a message includes a message body encoded with the chunked
transfer coding and the sender desires to send metadata in the form of
trailer fields at the end of the message, the sender &SHOULD; send a
Trailer header field before the message body to indicate
which fields will be present in the trailers. This allows the recipient
to prepare for receipt of that metadata before it starts processing the body,
which is useful if the message is being streamed and the recipient wishes
to confirm an integrity check on the fly.
Trailer = 1#field-name
If no Trailer header field is present, the sender of a
chunked message body &SHOULD; send an empty trailer.
A server &MUST; send an empty trailer with the chunked transfer coding
unless at least one of the following is true:
the request included a TE header field that indicates
"trailers" is acceptable in the transfer coding of the response, as
described in ; or,the trailer fields consist entirely of optional metadata and the
recipient could use the message (in a manner acceptable to the server where
the field originated) without receiving that metadata. In other words,
the server that generated the header field is willing to accept the
possibility that the trailer fields might be silently discarded along
the path to the client.
The above requirement prevents the need for an infinite buffer when a
message is being received by an HTTP/1.1 (or later) proxy and forwarded to
an HTTP/1.0 recipient.
A process for decoding the chunked transfer coding
can be represented in pseudo-code as:
length := 0
read chunk-size, chunk-ext (if any) and CRLF
while (chunk-size > 0) {
read chunk-data and CRLF
append chunk-data to decoded-body
length := length + chunk-size
read chunk-size and CRLF
}
read header-field
while (header-field not empty) {
append header-field to existing header fields
read header-field
}
Content-Length := length
Remove "chunked" from Transfer-Encoding
Remove Trailer from existing header fields
All recipients &MUST; be able to receive and decode the
chunked transfer coding and &MUST; ignore chunk-ext extensions
they do not understand.
The codings defined below can be used to compress the payload of a
message.
The "compress" format is produced by the common UNIX file compression
program "compress". This format is an adaptive Lempel-Ziv-Welch
coding (LZW). Recipients &SHOULD; consider "x-compress" to be
equivalent to "compress".
The "deflate" format is defined as the "deflate" compression mechanism
(described in ) used inside the "zlib"
data format ().
&Note; Some incorrect implementations send the "deflate"
compressed data without the zlib wrapper.
The "gzip" format is produced by the file compression program
"gzip" (GNU zip), as described in . This format is a
Lempel-Ziv coding (LZ77) with a 32 bit CRC.
Recipients &SHOULD; consider "x-gzip" to be equivalent to "gzip".
The "TE" header field in a request indicates what transfer codings,
besides chunked, the client is willing to accept in response, and
whether or not the client is willing to accept trailer fields in a
chunked transfer coding.
The TE field-value consists of a comma-separated list of transfer coding
names, each allowing for optional parameters (as described in
), and/or the keyword "trailers".
Clients &MUST-NOT; send the chunked transfer coding name in TE;
chunked is always acceptable for HTTP/1.1 recipients.
TE = #t-codingst-codings = "trailers" / ( transfer-coding [ t-ranking ] )
t-ranking = OWS ";" OWS "q=" rankrank = ( "0" [ "." 0*3DIGIT ] )
/ ( "1" [ "." 0*3("0") ] )
Three examples of TE use are below.
TE: deflate
TE:
TE: trailers, deflate;q=0.5
The presence of the keyword "trailers" indicates that the client is
willing to accept trailer fields in a chunked transfer coding,
as defined in , on behalf of itself and
any downstream clients. For chained requests, this implies that either:
(a) all downstream clients are willing to accept trailer fields in the
forwarded response; or,
(b) the client will attempt to buffer the response on behalf of downstream
recipients.
Note that HTTP/1.1 does not define any means to limit the size of a
chunked response such that a client can be assured of buffering the
entire response.
When multiple transfer codings are acceptable, the client &MAY; rank the
codings by preference using a case-insensitive "q" parameter (similar to
the qvalues used in content negotiation fields, &qvalue;). The rank value
is a real number in the range 0 through 1, where 0.001 is the least
preferred and 1 is the most preferred; a value of 0 means "not acceptable".
If the TE field-value is empty or if no TE field is present, the only
acceptable transfer coding is chunked. A message with no transfer coding
is always acceptable.
Since the TE header field only applies to the immediate connection,
a sender of TE &MUST; also send a "TE" connection option within the
Connection header field ()
in order to prevent the TE field from being forwarded by intermediaries
that do not support its semantics.
HTTP request message routing is determined by each client based on the
target resource, the client's proxy configuration, and
establishment or reuse of an inbound connection. The corresponding
response routing follows the same connection chain back to the client.
HTTP is used in a wide variety of applications, ranging from
general-purpose computers to home appliances. In some cases,
communication options are hard-coded in a client's configuration.
However, most HTTP clients rely on the same resource identification
mechanism and configuration techniques as general-purpose Web browsers.
HTTP communication is initiated by a user agent for some purpose.
The purpose is a combination of request semantics, which are defined in
, and a target resource upon which to apply those
semantics. A URI reference () is typically used as
an identifier for the "target resource", which a user agent
would resolve to its absolute form in order to obtain the
"target URI". The target URI
excludes the reference's fragment identifier component, if any,
since fragment identifiers are reserved for client-side processing
().
Once the target URI is determined, a client needs to decide whether
a network request is necessary to accomplish the desired semantics and,
if so, where that request is to be directed.
If the client has a response cache and the request semantics can be
satisfied by a cache (), then the request is
usually directed to the cache first.
If the request is not satisfied by a cache, then a typical client will
check its configuration to determine whether a proxy is to be used to
satisfy the request. Proxy configuration is implementation-dependent,
but is often based on URI prefix matching, selective authority matching,
or both, and the proxy itself is usually identified by an "http" or
"https" URI. If a proxy is applicable, the client connects inbound by
establishing (or reusing) a connection to that proxy.
If no proxy is applicable, a typical client will invoke a handler routine,
usually specific to the target URI's scheme, to connect directly
to an authority for the target resource. How that is accomplished is
dependent on the target URI scheme and defined by its associated
specification, similar to how this specification defines origin server
access for resolution of the "http" () and
"https" () schemes.
HTTP requirements regarding connection management are defined in
.
Once an inbound connection is obtained,
the client sends an HTTP request message ()
with a request-target derived from the target URI.
There are four distinct formats for the request-target, depending on both
the method being requested and whether the request is to a proxy.
request-target = origin-form
/ absolute-form
/ authority-form
/ asterisk-formorigin-form = absolute-path [ "?" query ]
absolute-form = absolute-URIauthority-form = authorityasterisk-form = "*"
origin-form
The most common form of request-target is the origin-form.
When making a request directly to an origin server, other than a CONNECT
or server-wide OPTIONS request (as detailed below),
a client &MUST; send only the absolute path and query components of
the target URI as the request-target.
If the target URI's path component is empty, then the client &MUST; send
"/" as the path within the origin-form of request-target.
A Host header field is also sent, as defined in
, containing the target URI's
authority component (excluding any userinfo).
For example, a client wishing to retrieve a representation of the resource
identified as
http://www.example.org/where?q=now
directly from the origin server would open (or reuse) a TCP connection
to port 80 of the host "www.example.org" and send the lines:
GET /where?q=now HTTP/1.1
Host: www.example.org
followed by the remainder of the request message.
absolute-form
When making a request to a proxy, other than a CONNECT or server-wide
OPTIONS request (as detailed below), a client &MUST; send the target URI
in absolute-form as the request-target.
The proxy is requested to either service that request from a valid cache,
if possible, or make the same request on the client's behalf to either
the next inbound proxy server or directly to the origin server indicated
by the request-target. Requirements on such "forwarding" of messages are
defined in .
An example absolute-form of request-line would be:
GET http://www.example.org/pub/WWW/TheProject.html HTTP/1.1
To allow for transition to the absolute-form for all requests in some
future version of HTTP, HTTP/1.1 servers &MUST; accept the absolute-form
in requests, even though HTTP/1.1 clients will only send them in requests
to proxies.
authority-form
The authority-form of request-target is only used for CONNECT requests
(&CONNECT;). When making a CONNECT request to establish a tunnel through
one or more proxies, a client &MUST; send only the target URI's
authority component (excluding any userinfo) as the request-target.
For example,
CONNECT www.example.com:80 HTTP/1.1
asterisk-form
The asterisk-form of request-target is only used for a server-wide
OPTIONS request (&OPTIONS;). When a client wishes to request OPTIONS
for the server as a whole, as opposed to a specific named resource of
that server, the client &MUST; send only "*" (%x2A) as the request-target.
For example,
OPTIONS * HTTP/1.1
If a proxy receives an OPTIONS request with an absolute-form of
request-target in which the URI has an empty path and no query component,
then the last proxy on the request chain &MUST; send a request-target
of "*" when it forwards the request to the indicated origin server.
For example, the request
OPTIONS http://www.example.org:8001 HTTP/1.1
would be forwarded by the final proxy as
OPTIONS * HTTP/1.1
Host: www.example.org:8001
after connecting to port 8001 of host "www.example.org".
The "Host" header field in a request provides the host and port
information from the target URI, enabling the origin
server to distinguish among resources while servicing requests
for multiple host names on a single IP address. Since the Host
field-value is critical information for handling a request, it
&SHOULD; be sent as the first header field following the request-line.
Host = uri-host [ ":" port ] ;
A client &MUST; send a Host header field in all HTTP/1.1 request
messages. If the target URI includes an authority component, then
the Host field-value &MUST; be identical to that authority component
after excluding any userinfo ().
If the authority component is missing or undefined for the target URI,
then the Host header field &MUST; be sent with an empty field-value.
For example, a GET request to the origin server for
<http://www.example.org/pub/WWW/> would begin with:
GET /pub/WWW/ HTTP/1.1
Host: www.example.org
The Host header field &MUST; be sent in an HTTP/1.1 request even
if the request-target is in the absolute-form, since this
allows the Host information to be forwarded through ancient HTTP/1.0
proxies that might not have implemented Host.
When a proxy receives a request with an absolute-form of
request-target, the proxy &MUST; ignore the received
Host header field (if any) and instead replace it with the host
information of the request-target. If the proxy forwards the request,
it &MUST; generate a new Host field-value based on the received
request-target rather than forward the received Host field-value.
Since the Host header field acts as an application-level routing
mechanism, it is a frequent target for malware seeking to poison
a shared cache or redirect a request to an unintended server.
An interception proxy is particularly vulnerable if it relies on
the Host field-value for redirecting requests to internal
servers, or for use as a cache key in a shared cache, without
first verifying that the intercepted connection is targeting a
valid IP address for that host.
A server &MUST; respond with a 400 (Bad Request) status code
to any HTTP/1.1 request message that lacks a Host header field and
to any request message that contains more than one Host header field
or a Host header field with an invalid field-value.
A server that receives an HTTP request message &MUST; reconstruct
the user agent's original target URI, based on the pieces of information
learned from the request-target, Host header field, and
connection context, in order to identify the intended target resource and
properly service the request. The URI derived from this reconstruction
process is referred to as the "effective request URI".
For a user agent, the effective request URI is the target URI.
If the request-target is in absolute-form, then the effective request URI
is the same as the request-target. Otherwise, the effective request URI
is constructed as follows.
If the request is received over a TLS-secured TCP connection,
then the effective request URI's scheme is "https"; otherwise, the
scheme is "http".
If the request-target is in authority-form, then the effective
request URI's authority component is the same as the request-target.
Otherwise, if a Host header field is supplied with a
non-empty field-value, then the authority component is the same as the
Host field-value. Otherwise, the authority component is the concatenation of
the default host name configured for the server, a colon (":"), and the
connection's incoming TCP port number in decimal form.
If the request-target is in authority-form or asterisk-form, then the
effective request URI's combined path and query component is empty.
Otherwise, the combined path and query component is the same as the
request-target.
The components of the effective request URI, once determined as above,
can be combined into absolute-URI form by concatenating the scheme,
"://", authority, and combined path and query component.
Example 1: the following message received over an insecure TCP connection
GET /pub/WWW/TheProject.html HTTP/1.1
Host: www.example.org:8080
has an effective request URI of
http://www.example.org:8080/pub/WWW/TheProject.html
Example 2: the following message received over a TLS-secured TCP connection
OPTIONS * HTTP/1.1
Host: www.example.org
has an effective request URI of
https://www.example.org
An origin server that does not allow resources to differ by requested
host &MAY; ignore the Host field-value and instead replace it
with a configured server name when constructing the effective request URI.
Recipients of an HTTP/1.0 request that lacks a Host header
field &MAY; attempt to use heuristics (e.g., examination of the URI path for
something unique to a particular host) in order to guess the
effective request URI's authority component.
HTTP does not include a request identifier for associating a given
request message with its corresponding one or more response messages.
Hence, it relies on the order of response arrival to correspond exactly
to the order in which requests are made on the same connection.
More than one response message per request only occurs when one or more
informational responses (1xx, see &status-1xx;) precede a
final response to the same request.
A client that has more than one outstanding request on a connection &MUST;
maintain a list of outstanding requests in the order sent and &MUST;
associate each received response message on that connection to the highest
ordered request that has not yet received a final (non-1xx)
response.
As described in , intermediaries can serve
a variety of roles in the processing of HTTP requests and responses.
Some intermediaries are used to improve performance or availability.
Others are used for access control or to filter content.
Since an HTTP stream has characteristics similar to a pipe-and-filter
architecture, there are no inherent limits to the extent an intermediary
can enhance (or interfere) with either direction of the stream.
Intermediaries that forward a message &MUST; implement the
Connection header field, as specified in
, to exclude fields that are only
intended for the incoming connection.
In order to avoid request loops, a proxy that forwards requests to other
proxies &MUST; be able to recognize and exclude all of its own server
names, including any aliases, local variations, or literal IP addresses.
The "Via" header field &MUST; be sent by a proxy or gateway in forwarded
messages to indicate the intermediate protocols and recipients between the
user agent and the server on requests, and between the origin server and
the client on responses. It is analogous to the "Received" field
used by email systems ().
Via is used in HTTP for tracking message forwards,
avoiding request loops, and identifying the protocol capabilities of
all senders along the request/response chain.
Via = 1#( received-protocolRWSreceived-by
[ RWScomment ] )
received-protocol = [ protocol-name "/" ] protocol-version
; see received-by = ( uri-host [ ":" port ] ) / pseudonympseudonym = token
The received-protocol indicates the protocol version of the message
received by the server or client along each segment of the
request/response chain. The received-protocol version is appended to
the Via field value when the message is forwarded so that information
about the protocol capabilities of upstream applications remains
visible to all recipients.
The protocol-name is excluded if and only if it would be "HTTP". The
received-by field is normally the host and optional port number of a
recipient server or client that subsequently forwarded the message.
However, if the real host is considered to be sensitive information,
it &MAY; be replaced by a pseudonym. If the port is not given, it &MAY;
be assumed to be the default port of the received-protocol.
Multiple Via field values represent each proxy or gateway that has
forwarded the message. Each recipient &MUST; append its information
such that the end result is ordered according to the sequence of
forwarding applications.
Comments &MAY; be used in the Via header field to identify the software
of each recipient, analogous to the User-Agent and
Server header fields. However, all comments in the Via field
are optional and &MAY; be removed by any recipient prior to forwarding the
message.
For example, a request message could be sent from an HTTP/1.0 user
agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
forward the request to a public proxy at p.example.net, which completes
the request by forwarding it to the origin server at www.example.com.
The request received by www.example.com would then have the following
Via header field:
Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)
A proxy or gateway used as a portal through a network firewall
&SHOULD-NOT; forward the names and ports of hosts within the firewall
region unless it is explicitly enabled to do so. If not enabled, the
received-by host of any host behind the firewall &SHOULD; be replaced
by an appropriate pseudonym for that host.
A proxy or gateway &MAY; combine an ordered subsequence of Via header
field entries into a single such entry if the entries have identical
received-protocol values. For example,
Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
could be collapsed to
Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
Senders &SHOULD-NOT; combine multiple entries unless they are all
under the same organizational control and the hosts have already been
replaced by pseudonyms. Senders &MUST-NOT; combine entries that
have different received-protocol values.
Some intermediaries include features for transforming messages and their
payloads. A transforming proxy might, for example, convert between image
formats in order to save cache space or to reduce the amount of traffic on
a slow link. However, operational problems might occur when these
transformations are applied to payloads intended for critical applications,
such as medical imaging or scientific data analysis, particularly when
integrity checks or digital signatures are used to ensure that the payload
received is identical to the original.
If a proxy receives a request-target with a host name that is not a
fully qualified domain name, it &MAY; add its own domain to the host name
it received when forwarding the request. A proxy &MUST-NOT; change the
host name if it is a fully qualified domain name.
A proxy &MUST-NOT; modify the "absolute-path" and "query" parts of the
received request-target when forwarding it to the next inbound server,
except as noted above to replace an empty path with "/" or "*".
A proxy &MUST-NOT; modify header fields that provide information about the
end points of the communication chain, the resource state, or the selected
representation. A proxy &MAY; change the message body through application
or removal of a transfer coding ().
A non-transforming proxy &MUST; preserve the message payload (&payload;).
A transforming proxy &MUST; preserve the payload of a message that
contains the no-transform cache-control directive.
A transforming proxy &MAY; transform the payload of a message
that does not contain the no-transform cache-control directive;
if the payload is transformed, the transforming proxy &MUST; add a
Warning 214 (Transformation applied) header field if one does not
already appear in the message (see &header-warning;).
HTTP messaging is independent of the underlying transport or
session-layer connection protocol(s). HTTP only presumes a reliable
transport with in-order delivery of requests and the corresponding
in-order delivery of responses. The mapping of HTTP request and
response structures onto the data units of an underlying transport
protocol is outside the scope of this specification.
As described in , the specific
connection protocols to be used for an HTTP interaction are determined by
client configuration and the target URI.
For example, the "http" URI scheme
() indicates a default connection of TCP
over IP, with a default TCP port of 80, but the client might be
configured to use a proxy via some other connection, port, or protocol.
HTTP implementations are expected to engage in connection management,
which includes maintaining the state of current connections,
establishing a new connection or reusing an existing connection,
processing messages received on a connection, detecting connection
failures, and closing each connection.
Most clients maintain multiple connections in parallel, including
more than one connection per server endpoint.
Most servers are designed to maintain thousands of concurrent connections,
while controlling request queues to enable fair use and detect
denial of service attacks.
The "Connection" header field allows the sender to indicate desired
control options for the current connection. In order to avoid confusing
downstream recipients, a proxy or gateway &MUST; remove or replace any
received connection options before forwarding the message.
When a header field aside from Connection is used to supply control
information for or about the current connection, the sender &MUST; list
the corresponding field-name within the "Connection" header field.
A proxy or gateway &MUST; parse a received Connection
header field before a message is forwarded and, for each
connection-option in this field, remove any header field(s) from
the message with the same name as the connection-option, and then
remove the Connection header field itself (or replace it with the
intermediary's own connection options for the forwarded message).
Hence, the Connection header field provides a declarative way of
distinguishing header fields that are only intended for the
immediate recipient ("hop-by-hop") from those fields that are
intended for all recipients on the chain ("end-to-end"), enabling the
message to be self-descriptive and allowing future connection-specific
extensions to be deployed without fear that they will be blindly
forwarded by older intermediaries.
The Connection header field's value has the following grammar:
Connection = 1#connection-optionconnection-option = token
Connection options are case-insensitive.
A sender &MUST-NOT; send a connection option corresponding to a header
field that is intended for all recipients of the payload.
For example, Cache-Control is never appropriate as a
connection option (&header-cache-control;).
The connection options do not have to correspond to a header field
present in the message, since a connection-specific header field
might not be needed if there are no parameters associated with that
connection option. Recipients that trigger certain connection
behavior based on the presence of connection options &MUST; do so
based on the presence of the connection-option rather than only the
presence of the optional header field. In other words, if the
connection option is received as a header field but not indicated
within the Connection field-value, then the recipient &MUST; ignore
the connection-specific header field because it has likely been
forwarded by an intermediary that is only partially conformant.
When defining new connection options, specifications ought to
carefully consider existing deployed header fields and ensure
that the new connection option does not share the same name as
an unrelated header field that might already be deployed.
Defining a new connection option essentially reserves that potential
field-name for carrying additional information related to the
connection option, since it would be unwise for senders to use
that field-name for anything else.
The "close" connection option is defined for a
sender to signal that this connection will be closed after completion of
the response. For example,
Connection: close
in either the request or the response header fields indicates that
the connection &MUST; be closed after the current request/response
is complete ().
A client that does not support persistent connections &MUST;
send the "close" connection option in every request message.
A server that does not support persistent connections &MUST;
send the "close" connection option in every response message that
does not have a 1xx (Informational) status code.
It is beyond the scope of this specification to describe how connections
are established via various transport or session-layer protocols.
Each connection applies to only one transport link.
HTTP/1.1 defaults to the use of "persistent connections",
allowing multiple requests and responses to be carried over a single
connection. The "close" connection-option is used to signal
that a connection will not persist after the current request/response.
HTTP implementations &SHOULD; support persistent connections.
A recipient determines whether a connection is persistent or not based on
the most recently received message's protocol version and
Connection header field (if any):
If the close connection option is present, the
connection will not persist after the current response; else,If the received protocol is HTTP/1.1 (or later), the connection will
persist after the current response; else,If the received protocol is HTTP/1.0, the "keep-alive"
connection option is present, the recipient is not a proxy, and
the recipient wishes to honor the HTTP/1.0 "keep-alive" mechanism,
the connection will persist after the current response; otherwise,The connection will close after the current response.
A server &MAY; assume that an HTTP/1.1 client intends to maintain a
persistent connection until a close connection option
is received in a request.
A client &MAY; reuse a persistent connection until it sends or receives
a close connection option or receives an HTTP/1.0 response
without a "keep-alive" connection option.
In order to remain persistent, all messages on a connection &MUST;
have a self-defined message length (i.e., one not defined by closure
of the connection), as described in .
A server &MUST; read the entire request message body or close
the connection after sending its response, since otherwise the
remaining data on a persistent connection would be misinterpreted
as the next request. Likewise,
a client &MUST; read the entire response message body if it intends
to reuse the same connection for a subsequent request.
A proxy server &MUST-NOT; maintain a persistent connection with an
HTTP/1.0 client (see for
information and discussion of the problems with the Keep-Alive header field
implemented by many HTTP/1.0 clients).
Clients and servers &SHOULD-NOT; assume that a persistent connection is
maintained for HTTP versions less than 1.1 unless it is explicitly
signaled.
See
for more information on backward compatibility with HTTP/1.0 clients.
Connections can be closed at any time, with or without intention.
Implementations ought to anticipate the need to recover
from asynchronous close events.
When an inbound connection is closed prematurely, a client &MAY; open a new
connection and automatically retransmit an aborted sequence of requests if
all of those requests have idempotent methods (&idempotent-methods;).
A proxy &MUST-NOT; automatically retry non-idempotent requests.
A user agent &MUST-NOT; automatically retry a request with a non-idempotent
method unless it has some means to know that the request semantics are
actually idempotent, regardless of the method, or some means to detect that
the original request was never applied. For example, a user agent that
knows (through design or configuration) that a POST request to a given
resource is safe can repeat that request automatically.
Likewise, a user agent designed specifically to operate on a version
control repository might be able to recover from partial failure conditions
by checking the target resource revision(s) after a failed connection,
reverting or fixing any changes that were partially applied, and then
automatically retrying the requests that failed.
An automatic retry &SHOULD-NOT; be repeated if it fails.
A client that supports persistent connections &MAY; "pipeline"
its requests (i.e., send multiple requests without waiting for each
response). A server &MAY; process a sequence of pipelined requests in
parallel if they all have safe methods (&safe-methods;), but &MUST; send
the corresponding responses in the same order that the requests were
received.
A client that pipelines requests &MUST; be prepared to retry those
requests if the connection closes before it receives all of the
corresponding responses. A client that assumes a persistent connection and
pipelines immediately after connection establishment &MUST-NOT; pipeline
on a retry connection until it knows the connection is persistent.
Idempotent methods (&idempotent-methods;) are significant to pipelining
because they can be automatically retried after a connection failure.
A user agent &SHOULD-NOT; pipeline requests after a non-idempotent method
until the final response status code for that method has been received,
unless the user agent has a means to detect and recover from partial
failure conditions involving the pipelined sequence.
An intermediary that receives pipelined requests &MAY; pipeline those
requests when forwarding them inbound, since it can rely on the outbound
user agent(s) to determine what requests can be safely pipelined. If the
inbound connection fails before receiving a response, the pipelining
intermediary &MAY; attempt to retry a sequence of requests that have yet
to receive a response if the requests all have idempotent methods;
otherwise, the pipelining intermediary &SHOULD; forward any received
responses and then close the corresponding outbound connection(s) so that
the outbound user agent(s) can recover accordingly.
Clients &SHOULD; limit the number of simultaneous
connections that they maintain to a given server.
Previous revisions of HTTP gave a specific number of connections as a
ceiling, but this was found to be impractical for many applications. As a
result, this specification does not mandate a particular maximum number of
connections, but instead encourages clients to be conservative when opening
multiple connections.
Multiple connections are typically used to avoid the "head-of-line
blocking" problem, wherein a request that takes significant server-side
processing and/or has a large payload blocks subsequent requests on the
same connection. However, each connection consumes server resources.
Furthermore, using multiple connections can cause undesirable side effects
in congested networks.
Note that servers might reject traffic that they deem abusive, including an
excessive number of connections from a client.
Servers will usually have some time-out value beyond which they will
no longer maintain an inactive connection. Proxy servers might make
this a higher value since it is likely that the client will be making
more connections through the same server. The use of persistent
connections places no requirements on the length (or existence) of
this time-out for either the client or the server.
When a client or server wishes to time-out it &SHOULD; issue a graceful
close on the transport connection. Clients and servers &SHOULD; both
constantly watch for the other side of the transport close, and
respond to it as appropriate. If a client or server does not detect
the other side's close promptly it could cause unnecessary resource
drain on the network.
A client, server, or proxy &MAY; close the transport connection at any
time. For example, a client might have started to send a new request
at the same time that the server has decided to close the "idle"
connection. From the server's point of view, the connection is being
closed while it was idle, but from the client's point of view, a
request is in progress.
Servers &SHOULD; maintain persistent connections and allow the underlying
transport's flow control mechanisms to resolve temporary overloads, rather
than terminate connections with the expectation that clients will retry.
The latter technique can exacerbate network congestion.
A client sending a message body &SHOULD; monitor
the network connection for an error status code while it is transmitting
the request. If the client sees an error status code, it &SHOULD;
immediately cease transmitting the body and close the connection.
The Connection header field
() provides a "close"
connection option that a sender &SHOULD; send when it wishes to close
the connection after the current request/response pair.
A client that sends a close connection option &MUST-NOT;
send further requests on that connection (after the one containing
close) and &MUST; close the connection after reading the
final response message corresponding to this request.
A server that receives a close connection option &MUST;
initiate a lingering close (see below) of the connection after it sends the
final response to the request that contained close.
The server &SHOULD; send a close connection option
in its final response on that connection. The server &MUST-NOT; process
any further requests received on that connection.
A server that sends a close connection option &MUST;
initiate a lingering close of the connection after it sends the
response containing close. The server &MUST-NOT; process
any further requests received on that connection.
A client that receives a close connection option &MUST;
cease sending requests on that connection and close the connection
after reading the response message containing the close; if additional
pipelined requests had been sent on the connection, the client &SHOULD;
assume that they will not be processed by the server.
If a server performs an immediate close of a TCP connection, there is a
significant risk that the client will not be able to read the last HTTP
response. If the server receives additional data from the client on a
fully-closed connection, such as another request that was sent by the
client before receiving the server's response, the server's TCP stack will
send a reset packet to the client; unfortunately, the reset packet might
erase the client's unacknowledged input buffers before they can be read
and interpreted by the client's HTTP parser.
To avoid the TCP reset problem, a server can perform a lingering close on a
connection by closing only the write side of the read/write connection
(a half-close) and continuing to read from the connection until the
connection is closed by the client or the server is reasonably certain
that its own TCP stack has received the client's acknowledgement of the
packet(s) containing the server's last response. It is then safe for the
server to fully close the connection.
It is unknown whether the reset problem is exclusive to TCP or might also
be found in other transport connection protocols.
The "Upgrade" header field is intended to provide a simple mechanism
for transitioning from HTTP/1.1 to some other protocol on the same
connection. A client &MAY; send a list of protocols in the Upgrade
header field of a request to invite the server to switch to one or
more of those protocols before sending the final response.
A server &MUST; send an Upgrade header field in 101 (Switching
Protocols) responses to indicate which protocol(s) are being
switched to, and &MUST; send it in 426 (Upgrade Required)
responses to indicate acceptable protocols.
A server &MAY; send an Upgrade header field in any other response to
indicate that they might be willing to upgrade to one of the
specified protocols for a future request.
Upgrade = 1#protocolprotocol = protocol-name ["/" protocol-version]
protocol-name = tokenprotocol-version = token
For example,
Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
Upgrade eases the difficult transition between incompatible protocols by
allowing the client to initiate a request in the more commonly
supported protocol while indicating to the server that it would like
to use a "better" protocol if available (where "better" is determined
by the server, possibly according to the nature of the request method
or target resource).
Upgrade cannot be used to insist on a protocol change; its acceptance and
use by the server is optional. The capabilities and nature of the
application-level communication after the protocol change is entirely
dependent upon the new protocol chosen, although the first action
after changing the protocol &MUST; be a response to the initial HTTP
request that contained the Upgrade header field.
For example, if the Upgrade header field is received in a GET request
and the server decides to switch protocols, then it first responds
with a 101 (Switching Protocols) message in HTTP/1.1 and
then immediately follows that with the new protocol's equivalent of a
response to a GET on the target resource. This allows a connection to be
upgraded to protocols with the same semantics as HTTP without the
latency cost of an additional round-trip. A server &MUST-NOT; switch
protocols unless the received message semantics can be honored by the new
protocol; an OPTIONS request can be honored by any protocol.
When Upgrade is sent, a sender &MUST; also send a
Connection header field ()
that contains the "upgrade" connection option, in order to prevent Upgrade
from being accidentally forwarded by intermediaries that might not implement
the listed protocols. A server &MUST; ignore an Upgrade header field that
is received in an HTTP/1.0 request.
The Upgrade header field only applies to switching application-level
protocols on the existing connection; it cannot be used
to switch to a protocol on a different connection. For that purpose, it is
more appropriate to use a 3xx (Redirection) response
(&status-3xx;).
This specification only defines the protocol name "HTTP" for use by
the family of Hypertext Transfer Protocols, as defined by the HTTP
version rules of and future updates to this
specification. Additional tokens ought to be registered with IANA using the
registration procedure defined in .
HTTP header fields are registered within the Message Header Field Registry
maintained by IANA at
.
This document defines the following HTTP header fields, so their
associated registry entries shall be updated according to the permanent
registrations below:
Header Field NameProtocolStatusReferenceConnectionhttpstandardContent-LengthhttpstandardHosthttpstandardTEhttpstandardTrailerhttpstandardTransfer-EncodinghttpstandardUpgradehttpstandardViahttpstandard
Furthermore, the header field-name "Close" shall be registered as
"reserved", since using that name as an HTTP header field might
conflict with the "close" connection option of the "Connection"
header field ().
Header Field NameProtocolStatusReferenceClosehttpreserved
The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
IANA maintains the registry of URI Schemes at
.
This document defines the following URI schemes, so their
associated registry entries shall be updated according to the permanent
registrations below:
URI SchemeDescriptionReferencehttpHypertext Transfer ProtocolhttpsHypertext Transfer Protocol Secure
This document serves as the specification for the Internet media types
"message/http" and "application/http". The following is to be registered with
IANA (see ).
The message/http type can be used to enclose a single HTTP request or
response message, provided that it obeys the MIME restrictions for all
"message" types regarding line length and encodings.
message
http
none
version, msgtype
The HTTP-version number of the enclosed message
(e.g., "1.1"). If not present, the version can be
determined from the first line of the body.
The message type — "request" or "response". If not
present, the type can be determined from the first
line of the body.
only "7bit", "8bit", or "binary" are permitted
none
none
This specification (see ).
nonenonenone
See Authors Section.
COMMON
none
IESG
The application/http type can be used to enclose a pipeline of one or more
HTTP request or response messages (not intermixed).
application
http
none
version, msgtype
The HTTP-version number of the enclosed messages
(e.g., "1.1"). If not present, the version can be
determined from the first line of the body.
The message type — "request" or "response". If not
present, the type can be determined from the first
line of the body.
HTTP messages enclosed by this type
are in "binary" format; use of an appropriate
Content-Transfer-Encoding is required when
transmitted via E-mail.
none
none
This specification (see ).
nonenonenone
See Authors Section.
COMMON
none
IESG
The HTTP Transfer Coding Registry defines the name space for transfer
coding names.
Registrations &MUST; include the following fields:
NameDescriptionPointer to specification text
Names of transfer codings &MUST-NOT; overlap with names of content codings
(&content-codings;) unless the encoding transformation is identical, as
is the case for the compression codings defined in
.
Values to be added to this name space require IETF Review (see
), and &MUST;
conform to the purpose of transfer coding defined in this section.
Use of program names for the identification of encoding formats
is not desirable and is discouraged for future encodings.
The registry itself is maintained at
.
The HTTP Transfer Coding Registry shall be updated with the registrations
below:
NameDescriptionReferencechunkedTransfer in a series of chunkscompressUNIX "compress" program methoddeflate"deflate" compression mechanism () used inside
the "zlib" data format ()
gzipSame as GNU zip
The HTTP Upgrade Token Registry defines the name space for protocol-name
tokens used to identify protocols in the Upgrade header
field. Each registered protocol name is associated with contact information
and an optional set of specifications that details how the connection
will be processed after it has been upgraded.
Registrations happen on a "First Come First Served" basis (see
) and are subject to the
following rules:
A protocol-name token, once registered, stays registered forever.The registration &MUST; name a responsible party for the
registration.The registration &MUST; name a point of contact.The registration &MAY; name a set of specifications associated with
that token. Such specifications need not be publicly available.The registration &SHOULD; name a set of expected "protocol-version"
tokens associated with that token at the time of registration.The responsible party &MAY; change the registration at any time.
The IANA will keep a record of all such changes, and make them
available upon request.The IESG &MAY; reassign responsibility for a protocol token.
This will normally only be used in the case when a
responsible party cannot be contacted.
This registration procedure for HTTP Upgrade Tokens replaces that
previously defined in .
The HTTP Upgrade Token Registry shall be updated with the registration
below:
ValueDescriptionExpected Version TokensReferenceHTTPHypertext Transfer Protocolany DIGIT.DIGIT (e.g, "2.0")
The responsible party is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
This section is meant to inform developers, information providers, and
users of known security concerns relevant to HTTP/1.1 message syntax,
parsing, and routing.
HTTP clients rely heavily on the Domain Name Service (DNS), and are thus
generally prone to security attacks based on the deliberate misassociation
of IP addresses and DNS names not protected by DNSSEC. Clients need to be
cautious in assuming the validity of an IP number/DNS name association unless
the response is protected by DNSSEC ().
By their very nature, HTTP intermediaries are men-in-the-middle, and
represent an opportunity for man-in-the-middle attacks. Compromise of
the systems on which the intermediaries run can result in serious security
and privacy problems. Intermediaries have access to security-related
information, personal information about individual users and
organizations, and proprietary information belonging to users and
content providers. A compromised intermediary, or an intermediary
implemented or configured without regard to security and privacy
considerations, might be used in the commission of a wide range of
potential attacks.
Intermediaries that contain a shared cache are especially vulnerable
to cache poisoning attacks.
Implementers need to consider the privacy and security
implications of their design and coding decisions, and of the
configuration options they provide to operators (especially the
default configuration).
Users need to be aware that intermediaries are no more trustworthy than
the people who run them; HTTP itself cannot solve this problem.
Because HTTP uses mostly textual, character-delimited fields, attackers can
overflow buffers in implementations, and/or perform a Denial of Service
against implementations that accept fields with unlimited lengths.
To promote interoperability, this specification makes specific
recommendations for minimum size limits on request-line
()
and blocks of header fields (). These are
minimum recommendations, chosen to be supportable even by implementations
with limited resources; it is expected that most implementations will
choose substantially higher limits.
This specification also provides a way for servers to reject messages that
have request-targets that are too long (&status-414;) or request entities
that are too large (&status-4xx;).
Recipients &SHOULD; carefully limit the extent to which they read other
fields, including (but not limited to) request methods, response status
phrases, header field-names, and body chunks, so as to avoid denial of
service attacks without impeding interoperability.
HTTP does not define a specific mechanism for ensuring message integrity,
instead relying on the error-detection ability of underlying transport
protocols and the use of length or chunk-delimited framing to detect
completeness. Additional integrity mechanisms, such as hash functions or
digital signatures applied to the content, can be selectively added to
messages via extensible metadata header fields. Historically, the lack of
a single integrity mechanism has been justified by the informal nature of
most HTTP communication. However, the prevalence of HTTP as an information
access mechanism has resulted in its increasing use within environments
where verification of message integrity is crucial.
User agents are encouraged to implement configurable means for detecting
and reporting failures of message integrity such that those means can be
enabled within environments for which integrity is necessary. For example,
a browser being used to view medical history or drug interaction
information needs to indicate to the user when such information is detected
by the protocol to be incomplete, expired, or corrupted during transfer.
Such mechanisms might be selectively enabled via user agent extensions or
the presence of message integrity metadata in a response.
At a minimum, user agents ought to provide some indication that allows a
user to distinguish between a complete and incomplete response message
() when such verification is desired.
A server is in the position to save personal data about a user's requests
over time, which might identify their reading patterns or subjects of
interest. In particular, log information gathered at an intermediary
often contains a history of user agent interaction, across a multitude
of sites, that can be traced to individual users.
HTTP log information is confidential in nature; its handling is often
constrained by laws and regulations. Log information needs to be securely
stored and appropriate guidelines followed for its analysis.
Anonymization of personal information within individual entries helps,
but is generally not sufficient to prevent real log traces from being
re-identified based on correlation with other access characteristics.
As such, access traces that are keyed to a specific client should not
be published even if the key is pseudonymous.
To minimize the risk of theft or accidental publication, log information
should be purged of personally identifiable information, including
user identifiers, IP addresses, and user-provided query parameters,
as soon as that information is no longer necessary to support operational
needs for security, auditing, or fraud control.
This edition of HTTP/1.1 builds on the many contributions that went into
RFC 1945,
RFC 2068,
RFC 2145, and
RFC 2616, including
substantial contributions made by the previous authors, editors, and
working group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
and Paul J. Leach. Mark Nottingham oversaw this effort as working group chair.
Since 1999, the following contributors have helped improve the HTTP
specification by reporting bugs, asking smart questions, drafting or
reviewing text, and evaluating open issues:
Adam Barth,
Adam Roach,
Addison Phillips,
Adrian Chadd,
Adrien W. de Croy,
Alan Ford,
Alan Ruttenberg,
Albert Lunde,
Alek Storm,
Alex Rousskov,
Alexandre Morgaut,
Alexey Melnikov,
Alisha Smith,
Amichai Rothman,
Amit Klein,
Amos Jeffries,
Andreas Maier,
Andreas Petersson,
Anil Sharma,
Anne van Kesteren,
Anthony Bryan,
Asbjorn Ulsberg,
Ashok Kumar,
Balachander Krishnamurthy,
Barry Leiba,
Ben Laurie,
Benjamin Niven-Jenkins,
Bil Corry,
Bill Burke,
Bjoern Hoehrmann,
Bob Scheifler,
Boris Zbarsky,
Brett Slatkin,
Brian Kell,
Brian McBarron,
Brian Pane,
Brian Smith,
Bryce Nesbitt,
Cameron Heavon-Jones,
Carl Kugler,
Carsten Bormann,
Charles Fry,
Chris Newman,
Chris Weber,
Cyrus Daboo,
Dale Robert Anderson,
Dan Wing,
Dan Winship,
Daniel Stenberg,
Darrel Miller,
Dave Cridland,
Dave Crocker,
Dave Kristol,
David Booth,
David Singer,
David W. Morris,
Diwakar Shetty,
Dmitry Kurochkin,
Drummond Reed,
Duane Wessels,
Duncan Cragg,
Edward Lee,
Eliot Lear,
Eran Hammer-Lahav,
Eric D. Williams,
Eric J. Bowman,
Eric Lawrence,
Eric Rescorla,
Erik Aronesty,
Evan Prodromou,
Florian Weimer,
Frank Ellermann,
Fred Bohle,
Gabriel Montenegro,
Geoffrey Sneddon,
Gervase Markham,
Grahame Grieve,
Greg Wilkins,
Harald Tveit Alvestrand,
Harry Halpin,
Helge Hess,
Henrik Nordstrom,
Henry S. Thompson,
Henry Story,
Herbert van de Sompel,
Howard Melman,
Hugo Haas,
Ian Fette,
Ian Hickson,
Ido Safruti,
Ilya Grigorik,
Ingo Struck,
J. Ross Nicoll,
James H. Manger,
James Lacey,
James M. Snell,
Jamie Lokier,
Jan Algermissen,
Jeff Hodges (who came up with the term 'effective Request-URI'),
Jeff Walden,
Jeroen de Borst,
Jim Luther,
Joe D. Williams,
Joe Gregorio,
Joe Orton,
John C. Klensin,
John C. Mallery,
John Cowan,
John Kemp,
John Panzer,
John Schneider,
John Stracke,
John Sullivan,
Jonas Sicking,
Jonathan A. Rees,
Jonathan Billington,
Jonathan Moore,
Jonathan Rees,
Jonathan Silvera,
Jordi Ros,
Joris Dobbelsteen,
Josh Cohen,
Julien Pierre,
Jungshik Shin,
Justin Chapweske,
Justin Erenkrantz,
Justin James,
Kalvinder Singh,
Karl Dubost,
Keith Hoffman,
Keith Moore,
Ken Murchison,
Koen Holtman,
Konstantin Voronkov,
Kris Zyp,
Lisa Dusseault,
Maciej Stachowiak,
Marc Schneider,
Marc Slemko,
Mark Baker,
Mark Pauley,
Mark Watson,
Markus Isomaki,
Markus Lanthaler,
Martin J. Duerst,
Martin Musatov,
Martin Nilsson,
Martin Thomson,
Matt Lynch,
Matthew Cox,
Max Clark,
Michael Burrows,
Michael Hausenblas,
Mike Amundsen,
Mike Belshe,
Mike Kelly,
Mike Schinkel,
Miles Sabin,
Murray S. Kucherawy,
Mykyta Yevstifeyev,
Nathan Rixham,
Nicholas Shanks,
Nico Williams,
Nicolas Alvarez,
Nicolas Mailhot,
Noah Slater,
Pablo Castro,
Pat Hayes,
Patrick R. McManus,
Patrik Faltstrom,
Paul E. Jones,
Paul Hoffman,
Paul Marquess,
Peter Lepeska,
Peter Saint-Andre,
Peter Watkins,
Phil Archer,
Philippe Mougin,
Phillip Hallam-Baker,
Poul-Henning Kamp,
Preethi Natarajan,
Rajeev Bector,
Ray Polk,
Reto Bachmann-Gmuer,
Richard Cyganiak,
Robert Brewer,
Robert Collins,
Robert O'Callahan,
Robert Olofsson,
Robert Sayre,
Robert Siemer,
Robert de Wilde,
Roberto Javier Godoy,
Roberto Peon,
Roland Zink,
Ronny Widjaja,
S. Mike Dierken,
Salvatore Loreto,
Sam Johnston,
Sam Ruby,
Scott Lawrence (who maintained the original issues list),
Sean B. Palmer,
Shane McCarron,
Stefan Eissing,
Stefan Tilkov,
Stefanos Harhalakis,
Stephane Bortzmeyer,
Stephen Farrell,
Stephen Ludin,
Stuart Williams,
Subbu Allamaraju,
Subramanian Moonesamy,
Sylvain Hellegouarch,
Tapan Divekar,
Tatsuya Hayashi,
Ted Hardie,
Thomas Broyer,
Thomas Fossati,
Thomas Nordin,
Thomas Roessler,
Tim Bray,
Tim Morgan,
Tim Olsen,
Tobias Oberstein,
Tom Zhou,
Travis Snoozy,
Tyler Close,
Vincent Murphy,
Wenbo Zhu,
Werner Baumann,
Wilbur Streett,
Wilfredo Sanchez Vega,
William A. Rowe Jr.,
William Chan,
Willy Tarreau,
Xiaoshu Wang,
Yaron Goland,
Yngve Nysaeter Pettersen,
Yoav Nir,
Yogesh Bang,
Yutaka Oiwa,
Yves Lafon (long-time member of the editor team),
Zed A. Shaw, and
Zhong Yu.
See for additional
acknowledgements from prior revisions.
Hypertext Transfer Protocol (HTTP/1.1): Semantics and ContentAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.de1xx (Informational)1xx100 (Continue)101 (Switching Protocols)2xx (Successful)2xx200 (OK)204 (No Content)3xx (Redirection)3xx301 (Moved Permanently)4xx (Client Error)4xx400 (Bad Request)411 (Length Required)414 (URI Too Long)417 (Expectation Failed)426 (Upgrade Required)501 (Not Implemented)502 (Bad Gateway)505 (HTTP Version Not Supported)AllowContent-EncodingContent-LocationContent-TypeDateExpectLocationServerUser-AgentHypertext Transfer Protocol (HTTP/1.1): Conditional RequestsAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.de304 (Not Modified)ETagLast-ModifiedHypertext Transfer Protocol (HTTP/1.1): Range RequestsAdobe Systems Incorporatedfielding@gbiv.comWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deContent-RangeHypertext Transfer Protocol (HTTP/1.1): CachingAdobe Systems Incorporatedfielding@gbiv.comAkamaimnot@mnot.netgreenbytes GmbHjulian.reschke@greenbytes.deCache-ControlExpiresHypertext Transfer Protocol (HTTP/1.1): AuthenticationAdobe Systems Incorporatedfielding@gbiv.comgreenbytes GmbHjulian.reschke@greenbytes.deProxy-AuthenticateProxy-AuthorizationAugmented BNF for Syntax Specifications: ABNFBrandenburg InternetWorkingdcrocker@bbiw.netTHUS plc.paul.overell@thus.netKey words for use in RFCs to Indicate Requirement LevelsHarvard Universitysob@harvard.eduUniform Resource Identifier (URI): Generic SyntaxWorld Wide Web Consortiumtimbl@w3.orghttp://www.w3.org/People/Berners-Lee/Day Softwarefielding@gbiv.comhttp://roy.gbiv.com/Adobe Systems IncorporatedLMM@acm.orghttp://larry.masinter.net/Coded Character Set -- 7-bit American Standard Code for Information InterchangeAmerican National Standards InstituteZLIB Compressed Data Format Specification version 3.3Aladdin Enterprisesghost@aladdin.comDEFLATE Compressed Data Format Specification version 1.3Aladdin Enterprisesghost@aladdin.comGZIP file format specification version 4.3Aladdin Enterprisesghost@aladdin.comgzip@prep.ai.mit.edumadler@alumni.caltech.edughost@aladdin.comrandeg@alumni.rpi.edu
Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1
International Organization for StandardizationClassical versus Transparent IP Proxiesmchatel@pax.eunet.chHypertext Transfer Protocol -- HTTP/1.0MIT, Laboratory for Computer Sciencetimbl@w3.orgUniversity of California, Irvine, Department of Information and Computer Sciencefielding@ics.uci.eduW3 Consortium, MIT Laboratory for Computer Sciencefrystyk@w3.orgMultipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message BodiesInnosoft International, Inc.ned@innosoft.comFirst Virtual Holdingsnsb@nsb.fv.comMIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII TextUniversity of Tennesseemoore@cs.utk.eduHypertext Transfer Protocol -- HTTP/1.1University of California, Irvine, Department of Information and Computer Sciencefielding@ics.uci.eduMIT Laboratory for Computer Sciencejg@w3.orgDigital Equipment Corporation, Western Research Laboratorymogul@wrl.dec.comMIT Laboratory for Computer Sciencefrystyk@w3.orgMIT Laboratory for Computer Sciencetimbl@w3.orgUse and Interpretation of HTTP Version NumbersWestern Research Laboratorymogul@wrl.dec.comDepartment of Information and Computer Sciencefielding@ics.uci.eduMIT Laboratory for Computer Sciencejg@w3.orgW3 Consortiumfrystyk@w3.orgHypertext Transfer Protocol -- HTTP/1.1University of California, Irvinefielding@ics.uci.eduW3Cjg@w3.orgCompaq Computer Corporationmogul@wrl.dec.comMIT Laboratory for Computer Sciencefrystyk@w3.orgXerox Corporationmasinter@parc.xerox.comMicrosoft Corporationpaulle@microsoft.comW3Ctimbl@w3.orgUpgrading to TLS Within HTTP/1.14K Associates / UC Irvinerohit@4K-associates.comAgranat Systems, Inc.lawrence@agranat.comHTTP Over TLSRTFM, Inc.ekr@rtfm.comInternet Web Replication and Caching TaxonomyEquinix, Inc.UNINETTCacheFlow Inc.Registration Procedures for Message Header FieldsNine by NineGK-IETF@ninebynine.orgBEA Systemsmnot@pobox.comHP LabsJeffMogul@acm.orgDNS Security Introduction and RequirementsMedia Type Specifications and Registration ProceduresOraclened+ietf@mrochek.comjohn+ietf@jck.comAT&T Laboratoriestony+mtsuffix@maillennium.att.comGuidelines and Registration Procedures for New URI SchemesAT&T Laboratoriestony+urireg@maillennium.att.comQualcomm, Inc.hardie@qualcomm.comAdobe SystemsLMM@acm.orgSPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft WindowsGuidelines for Writing an IANA Considerations Section in RFCsIBMnarten@us.ibm.comGoogleHarald@Alvestrand.noThe Transport Layer Security (TLS) Protocol Version 1.2RTFM, Inc.Internet Message FormatQualcomm IncorporatedHTTP State Management Mechanism
University of California, Berkeley
abarth@eecs.berkeley.eduHTTP Cookies: Standards, Privacy, and Politics
HTTP has been in use by the World-Wide Web global information initiative
since 1990. The first version of HTTP, later referred to as HTTP/0.9,
was a simple protocol for hypertext data transfer across the Internet
with only a single request method (GET) and no metadata.
HTTP/1.0, as defined by , added a range of request
methods and MIME-like messaging that could include metadata about the data
transferred and modifiers on the request/response semantics. However,
HTTP/1.0 did not sufficiently take into consideration the effects of
hierarchical proxies, caching, the need for persistent connections, or
name-based virtual hosts. The proliferation of incompletely-implemented
applications calling themselves "HTTP/1.0" further necessitated a
protocol version change in order for two communicating applications
to determine each other's true capabilities.
HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
requirements that enable reliable implementations, adding only
those new features that will either be safely ignored by an HTTP/1.0
recipient or only sent when communicating with a party advertising
conformance with HTTP/1.1.
It is beyond the scope of a protocol specification to mandate
conformance with previous versions. HTTP/1.1 was deliberately
designed, however, to make supporting previous versions easy.
We would expect a general-purpose HTTP/1.1 server to understand
any valid request in the format of HTTP/1.0 and respond appropriately
with an HTTP/1.1 message that only uses features understood (or
safely ignored) by HTTP/1.0 clients. Likewise, we would expect
an HTTP/1.1 client to understand any valid HTTP/1.0 response.
Since HTTP/0.9 did not support header fields in a request,
there is no mechanism for it to support name-based virtual
hosts (selection of resource by inspection of the Host header
field). Any server that implements name-based virtual hosts
ought to disable support for HTTP/0.9. Most requests that
appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x
requests wherein a buggy client failed to properly encode
linear whitespace found in a URI reference and placed in
the request-target.
This section summarizes major differences between versions HTTP/1.0
and HTTP/1.1.
The requirements that clients and servers support the Host
header field (), report an error if it is
missing from an HTTP/1.1 request, and accept absolute URIs ()
are among the most important changes defined by HTTP/1.1.
Older HTTP/1.0 clients assumed a one-to-one relationship of IP
addresses and servers; there was no other established mechanism for
distinguishing the intended server of a request than the IP address
to which that request was directed. The Host header field was
introduced during the development of HTTP/1.1 and, though it was
quickly implemented by most HTTP/1.0 browsers, additional requirements
were placed on all HTTP/1.1 requests in order to ensure complete
adoption. At the time of this writing, most HTTP-based services
are dependent upon the Host header field for targeting requests.
In HTTP/1.0, each connection is established by the client prior to the
request and closed by the server after sending the response. However, some
implementations implement the explicitly negotiated ("Keep-Alive") version
of persistent connections described in .
Some clients and servers might wish to be compatible with these previous
approaches to persistent connections, by explicitly negotiating for them
with a "Connection: keep-alive" request header field. However, some
experimental implementations of HTTP/1.0 persistent connections are faulty;
for example, if an HTTP/1.0 proxy server doesn't understand
Connection, it will erroneously forward that header field
to the next inbound server, which would result in a hung connection.
One attempted solution was the introduction of a Proxy-Connection header
field, targeted specifically at proxies. In practice, this was also
unworkable, because proxies are often deployed in multiple layers, bringing
about the same problem discussed above.
As a result, clients are encouraged not to send the Proxy-Connection header
field in any requests.
Clients are also encouraged to consider the use of Connection: keep-alive
in requests carefully; while they can enable persistent connections with
HTTP/1.0 servers, clients using them need will need to monitor the
connection for "hung" requests (which indicate that the client ought stop
sending the header field), and this mechanism ought not be used by clients
at all when a proxy is being used.
HTTP/1.1 introduces the Transfer-Encoding header field
().
Transfer codings need to be decoded prior to forwarding an HTTP message
over a MIME-compliant protocol.
HTTP's approach to error handling has been explained.
()
The expectation to support HTTP/0.9 requests has been removed.
The term "Effective Request URI" has been introduced.
()
HTTP messages can be (and often are) buffered by implementations; despite
it sometimes being available as a stream, HTTP is fundamentally a
message-oriented protocol.
()
Minimum supported sizes for various protocol elements have been
suggested, to improve interoperability.
Header fields that span multiple lines ("line folding") are deprecated.
()
The HTTP-version ABNF production has been clarified to be case-sensitive.
Additionally, version numbers has been restricted to single digits, due
to the fact that implementations are known to handle multi-digit version
numbers incorrectly.
()
The HTTPS URI scheme is now defined by this specification; previously,
it was done in .
()
The HTTPS URI scheme implies end-to-end security.
()
Userinfo (i.e., username and password) are now disallowed in HTTP and
HTTPS URIs, because of security issues related to their transmission on the
wire.
()
Invalid whitespace around field-names is now required to be rejected,
because accepting it represents a security vulnerability.
()
The ABNF productions defining header fields now only list the field value.
()
Rules about implicit linear whitespace between certain grammar productions
have been removed; now whitespace is only allowed where specifically
defined in the ABNF.
()
The NUL octet is no longer allowed in comment and quoted-string text, and
handling of backslash-escaping in them has been clarified.
()
The quoted-pair rule no longer allows escaping control characters other than
HTAB.
()
Non-ASCII content in header fields and the reason phrase has been obsoleted
and made opaque (the TEXT rule was removed).
()
Bogus "Content-Length" header fields are now required to be
handled as errors by recipients.
()
The "identity" transfer coding token has been removed.
(Sections and
)
The algorithm for determining the message body length has been clarified
to indicate all of the special cases (e.g., driven by methods or status
codes) that affect it, and that new protocol elements cannot define such
special cases.
()
"multipart/byteranges" is no longer a way of determining message body length
detection.
()
CONNECT is a new, special case in determining message body length.
()
Chunk length does not include the count of the octets in the
chunk header and trailer.
()
Use of chunk extensions is deprecated, and line folding in them is
disallowed.
()
The segment + query components of RFC3986 have been used to define the
request-target, instead of abs_path from RFC 1808.
()
The asterisk form of the request-target is only allowed in the OPTIONS
method.
()
Exactly when "close" connection options have to be sent has been clarified.
()
"hop-by-hop" header fields are required to appear in the Connection header
field; just because they're defined as hop-by-hop in this specification
doesn't exempt them.
()
The limit of two connections per server has been removed.
()
An idempotent sequence of requests is no longer required to be retried.
()
The requirement to retry requests under certain circumstances when the
server prematurely closes the connection has been removed.
()
Some extraneous requirements about when servers are allowed to close
connections prematurely have been removed.
()
The semantics of the Upgrade header field is now defined in
responses other than 101 (this was incorporated from ).
()
Registration of Transfer Codings now requires IETF Review
()
The meaning of the "deflate" content coding has been clarified.
()
This specification now defines the Upgrade Token Registry, previously
defined in .
()
Empty list elements in list productions (e.g., a list header containing
", ,") have been deprecated.
()
Issues with the Keep-Alive and Proxy-Connection headers in requests
are pointed out, with use of the latter being discouraged altogether.
()
A #rule extension to the ABNF rules of is used to
improve readability in the definitions of some header field values.
A construct "#" is defined, similar to "*", for defining comma-delimited
lists of elements. The full form is "<n>#<m>element" indicating
at least <n> and at most <m> elements, each separated by a single
comma (",") and optional whitespace (OWS).
Thus,
1#element => element *( OWS "," OWS element )
and:
#element => [ 1#element ]
and for n >= 1 and m > 1:
<n>#<m>element => element <n-1>*<m-1>( OWS "," OWS element )
For compatibility with legacy list rules, recipients &SHOULD; accept empty
list elements. In other words, consumers would follow the list productions:
#element => [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
1#element => *( "," OWS ) element *( OWS "," [ OWS element ] )
Note that empty elements do not contribute to the count of elements present,
though.
For example, given these ABNF productions:
example-list = 1#example-list-elmt
example-list-elmt = token ; see
Then these are valid values for example-list (not including the double
quotes, which are present for delimitation only):
"foo,bar"
"foo ,bar,"
"foo , ,bar,charlie "
But these values would be invalid, as at least one non-empty element is
required:
""
","
", ,"
shows the collected ABNF, with the list rules
expanded as explained above.
BWS = OWS
Connection = *( "," OWS ) connection-option *( OWS "," [ OWS
connection-option ] )
Content-Length = 1*DIGIT
HTTP-message = start-line *( header-field CRLF ) CRLF [ message-body
]
HTTP-name = %x48.54.54.50 ; HTTP
HTTP-version = HTTP-name "/" DIGIT "." DIGIT
Host = uri-host [ ":" port ]
OWS = *( SP / HTAB )
RWS = 1*( SP / HTAB )
TE = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
Trailer = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
Transfer-Encoding = *( "," OWS ) transfer-coding *( OWS "," [ OWS
transfer-coding ] )
URI-reference = <URI-reference, defined in [RFC3986], Section 4.1>
Upgrade = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )
Via = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
comment ] ) ] )
absolute-URI = <absolute-URI, defined in [RFC3986], Section 4.3>
absolute-form = absolute-URI
absolute-path = 1*( "/" segment )
asterisk-form = "*"
attribute = token
authority = <authority, defined in [RFC3986], Section 3.2>
authority-form = authority
chunk = chunk-size [ chunk-ext ] CRLF chunk-data CRLF
chunk-data = 1*OCTET
chunk-ext = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
chunk-ext-name = token
chunk-ext-val = token / quoted-str-nf
chunk-size = 1*HEXDIG
chunked-body = *chunk last-chunk trailer-part CRLF
comment = "(" *( ctext / quoted-cpair / comment ) ")"
connection-option = token
ctext = HTAB / SP / %x21-27 ; '!'-'''
/ %x2A-5B ; '*'-'['
/ %x5D-7E ; ']'-'~'
/ obs-text
field-content = *( HTAB / SP / VCHAR / obs-text )
field-name = token
field-value = *( field-content / obs-fold )
header-field = field-name ":" OWS field-value BWS
http-URI = "http://" authority path-abempty [ "?" query ]
https-URI = "https://" authority path-abempty [ "?" query ]
last-chunk = 1*"0" [ chunk-ext ] CRLF
message-body = *OCTET
method = token
obs-fold = CRLF ( SP / HTAB )
obs-text = %x80-FF
origin-form = absolute-path [ "?" query ]
partial-URI = relative-part [ "?" query ]
path-abempty = <path-abempty, defined in [RFC3986], Section 3.3>
port = <port, defined in [RFC3986], Section 3.2.3>
protocol = protocol-name [ "/" protocol-version ]
protocol-name = token
protocol-version = token
pseudonym = token
qdtext = HTAB / SP / "!" / %x23-5B ; '#'-'['
/ %x5D-7E ; ']'-'~'
/ obs-text
qdtext-nf = HTAB / SP / "!" / %x23-5B ; '#'-'['
/ %x5D-7E ; ']'-'~'
/ obs-text
query = <query, defined in [RFC3986], Section 3.4>
quoted-cpair = "\" ( HTAB / SP / VCHAR / obs-text )
quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )
quoted-str-nf = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE
rank = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
reason-phrase = *( HTAB / SP / VCHAR / obs-text )
received-by = ( uri-host [ ":" port ] ) / pseudonym
received-protocol = [ protocol-name "/" ] protocol-version
relative-part = <relative-part, defined in [RFC3986], Section 4.2>
request-line = method SP request-target SP HTTP-version CRLF
request-target = origin-form / absolute-form / authority-form /
asterisk-form
segment = <segment, defined in [RFC3986], Section 3.3>
special = "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "\" /
DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
start-line = request-line / status-line
status-code = 3DIGIT
status-line = HTTP-version SP status-code SP reason-phrase CRLF
t-codings = "trailers" / ( transfer-coding [ t-ranking ] )
t-ranking = OWS ";" OWS "q=" rank
tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /
"^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
token = 1*tchar
trailer-part = *( header-field CRLF )
transfer-coding = "chunked" / "compress" / "deflate" / "gzip" /
transfer-extension
transfer-extension = token *( OWS ";" OWS transfer-parameter )
transfer-parameter = attribute BWS "=" BWS value
uri-host = <host, defined in [RFC3986], Section 3.2.2>
value = word
word = token / quoted-string
Changes up to the first Working Group Last Call draft are summarized
in .
Closed issues:
:
"Cite HTTPS URI scheme definition" (the spec now includes the HTTPs
scheme definition and thus updates RFC 2818)
:
"mention of 'proxies' in section about caches"
:
"use of ABNF terms from RFC 3986"
:
"editorial improvements to message length definition"
:
"Connection header field MUST vs SHOULD"
:
"editorial improvements to persistent connections section"
:
"URI normalization vs empty path"
:
"p1 feedback"
:
"is parsing OBS-FOLD mandatory?"
:
"HTTPS and Shared Caching"
:
"Requirements for recipients of ws between start-line and first header field"
:
"SP and HT when being tolerant"
:
"Message Parsing Strictness"
:
"'Render'"
:
"No-Transform"
:
"p2 editorial feedback"
:
"Content-Length SHOULD be sent"
:
"origin-form does not allow path starting with "//""
:
"ambiguity in part 1 example"